Hierarchy of State Routes 1-10 (or of 10 lowest numbered state highways)

[paulthemapguy]

IL-1 is the longest state route, so that has to come first.
IL-9 runs from Iowa to Indiana, spanning the entire state, serving as the main east-west route through Bloomington.
IL-3 is the westernmost state highway along the Mississippi River from just north of the Metro-East all the way to Cairo.
IL-4 is a long state route from Springfield along parts of US66 and continues south of the Metro-East.
IL-2 follows the Rock River from Sterling through Rockford to the Wisconsin border.  I'd say it's important.
IL-10 is semi-long from Champaign through Clinton and Lincoln to somewhere east of Havana.
IL-7 is pretty short but it heads through the suburbs, garnering more of an ADT.
IL-8 is pretty short, going through Peoria.
IL-6 is a freeway, but it's short and not very heavily traveled.
IL-5 is a small surface road fragment left behind by the christening of I-88.

I admittedly mainly based this on the routes' length and not much else.

[Mapmikey]

South Carolina, skipping US routes:

9 - major route across much of the top of the state
11 - mostly a modern route across the foothills for over 100 miles (Cherokee Foothills Scenic Hwy)
4 - connects Aiken to Orangeburg
5 - connects I-85 to Rock Hill
6 - proximity to 3 different lakes
3 - long but mostly rural
8 - moderate length in the Piedmont
10 - now a short route that is redundant of US 221 south of Greenwood
7 - short route in the Charleston metro.  This was much more important before I-526 came along to connect with US 17 south
2 - short highway through Cayce

[Evan_Th]

Washington State:

I-5 - definitely the most important on this list.
US 2 - the second-most-important of the three east-west backbones of the state (behind I-90)
SR 9 - bypass to northern I-5, and a decently major commuter route
SR 7 - major route out of south Tacoma
SR 3 - regionally major route on the Kitsap Peninsula
SR 8 - part of the Olympia-to-the-sea route that used to be US 410
SR 4 - another regionally major route along the north bank of the Columbia.
SR 6 - connector between I-5 and the coast.
SR 10 - a remnant of old US 10, but now a minor rural conector.

Bonus to make up a list of 10:
SR 11 - a decent coastal drive; I'd place it just above SR 6.

[Bickendan]

Oregon has a weird way of numbering its highways (where the highway number is usually different than the signed route number), but for this I'll do the 10 lowest route numbers. I'll start with including US and Interstate highways. Might make a second list with only state routes another time (but there are only 2 differences). So the candidates are: OR-3, I-5, OR-6, OR-7, OR-8, OR-10, OR-11, OR-18, OR-19, US-20.

1. I-5: Duh
2. US-20: Longest route on this list, and one of the longest in the whole state
3. OR-18: Provides a major link between Portland/Salem and the coast (Lincoln City)
4. OR-6: Connects Portland to Tillamook
5. OR-11: Short, but provides a somewhat important connection between Walla Walla and Pendleton
6. OR-8: Mostly urban/suburban connections in the Portland Metro. Not sure if it needs a state highway number, but it's an important road
7. OR-10: Similar to OR-8, but shorter
8. OR-7: Connects Baker City to Central Oregon, but that's about it.
9. OR-19: In the middle of nowhere, not terribly useful because of US-395 and US-97. Provides an alternate route from John Day to Portland, but US-26 the whole way is probably a better route.
10. OR-3: Tucked in the remote corner of the state, this route has very little use. Really only useful for the Lewiston to Enterprise route, as in almost nothing. You could argue that it's the best route between Baker City/La Grande and Lewiston, but Google Maps won't route along it, though it is notably shorter (I haven't been on this route in nearly 10 years (I think; I don't remember if we used it on our Wallowa Lake trip; knowing my Dad, it wouldn't surprise me if we did), before I could drive, so I don't remember it).

Since the Routes were already done, I'll do the Highways
1: Pacific Highway, I-5. The backbone of the state.
2: Columbia River Highway, I-84, US 30, and US 730. The best trans-Cascades route.
6: Old Oregon Trail, I-84 and US 30 east of US 30. Gives the connection between Idaho and east to Pendleton, the TriCities, and out to Portland via ORH 2 to the west.
4: The Dalles-California Highway, US 197 and US 97. Serves as a good alternate to ORH 1, and rising in prominance with the less than ideal upgrades through Bend, but still holds potential as a major corridor.
9: Oregon Coast Highway, US 101. Longest of the north-south highways, and its overlapping Route is the longest of the north-south Routes. Sections like Tillamook to Manzanita and Cannon Beach Jct through Seaside badly need attention.
7: Central Oregon Highway, US 20 from Bend to Vale, US 20, 26 (and OR 201) from Vale to Nyssa. Possibly the most desolate segment of US 20 in its trans-continental trek (though Craters of the Moon to Idaho Falls might eke it out).
5: John Day Highway, OR 19 and US 26 from OR 19 to US 20. As a corridor, one could question the point of it -- after all, why would someone go from Arlington to Vale? But, it serves to connect John Day with the John Day Fossil Beds, and runs roughly along the John Day River. Arlington and Vale end up being the logical end points.
8: Oregon-Washington Highway, OR 11. Pendleton toward Walla Walla.
10: Wallowa Lake Highway, OR 82. La Grande to Joseph. Honestly, this probably shouldn't have gotten a low numbered Highway number, but it might have been more important back in 1917 when it was designated.
3: Oswego Highway, OR 43. Portland to Oregon City. The shortest one of this group, and completely within the Portland Metro area. ODOT wants to turn it over to West Linn, Lake Oswego, and Portland, but the section of Macadam Ave between Lake Oswego and the Sellwood Bridge, and Macadam and Hood Avenues within the Ross Island Maze are probably preventing that. ORH 3 does have the distinction of being one of US 99's former routings.

[SkyPesos]

Ohio has been done already, but here's my own version, skipping US 6 and former OH 1:

OH 2 - Freeway for a good portion, Shunpike route for turnpike
OH 7 - Longest state route in the state, connects various cities along the Ohio river on the southeast and east of the state. Along with OH 11, forms a freeway corridor between Wheeling and Ashtabula.
OH 4 - Major highway between Cincinnati and Dayton, serving cities that I-75 bypassed. Freeway in Dayton, though it's mostly a 2 lane the rest of the way north to Sandusky.
OH 3 - Arterial in the 3C cities it goes through. Besides that, not really useful for long distance travel, as it parallels I-71. Also, the entire portion south of Columbus is concurrent with US 62 and US 22.
OH 8 - Freeway between Akron and the east side of the Cleveland metro. North of there, it's a local road, mostly concurrent with US 422 to downtown Cleveland.
OH 11 - Freeway for its entire length, though it's pretty lightly traveled outside of the Youngstown area.
OH 10 - Continues the US 20 freeway east of Oberlin to I-480. East of there, it's mostly a local road, though will eventually be extended using the Opportunity Corridor to University Circle.
OH 5 - Besides Warren, nothing notable on this route.
OH 9 - Mostly a rural 2 lane road in the eastern side of the state. You can tell it's not that important when Google maps mark it in a white color, opposed to the yellow most other state routes are marked in.
OH 12 - Pretty short minor road in the western part of the state. Like OH 9, it's mostly marked in white in rural areas.

[Dirt Roads]

I'm suprised nobody ever did North Carolina, where the low-numbered routes are fairly minor.  Here's the listing in numeric order.

US-1 
NC-2  NC-5/Pinehurst -to- US-1/Southern Pines (5.5 miles, all four-lane)
NC-3  US-601/Kannapolis -to- NC-152/Mooresville (27.0 miles)(designated in 2002 to commemorate Dale Earnhardt, Sr.)   
NC-4  US-301/Battleboro -to- US-158/Littleton (34.7 miles)
NC-5  NC-211/Pinehurst -to- Malcolm Blue Farmstead park entrance/Aberdeen (8.3 miles)
NC-6  I-40/Greensboro/Patterson Street/Exit 216 -to- I-40/Greensboro/Lee Street/Exit 224 (7.9 miles)(decommissioned 2004)
NC-7  US-29/US-74/Gastonia -to- US-321/Belmont (12.1 miles)
NC-8  US-52/New London -to- VA-8/Virginia State Line (94.6 miles)
NC-9  Montreat -to- SC-9/South Carolina Line (46.5 miles)
NC-10  I-40/Catawba/Oxford School Road/Exit 138 -to- NC-226/Polkville (48.3 miles)
NC-11  US-74/US-76/Freeman -to- US-158/Murfreesboro (193.2 miles)
NC-12  US-70/Sea Level -to- Corolla (148.0 miles)

First place is a no-brainer.  NC-12 runs the entire length of the Outer Banks, plus the entire length of Ocracoke Island, and incorporates the two popular ferries.

Second place is NC-11.  It serves as major route from Kinston to Greenville and is superior to US-258 along much of the route.

Third place is a toss-up between NC-2 and NC-10.  The former is the charming gateway to both Pinehurst and Southern Pines.  The latter used to be the main gateway to Claremont and Newton from points east.

Fifth place?  How about NC-4, historically important as the north end of the former Green I-95 Business Loop through Rocky Mount and Wilson.  That particular section was even more important when southbound I-95 ended there prior to 1979, but NC-4 wasn't designated on that part of the route until when Green I-95 was decommissioned.  The rest of NC-4 probably falls down to the bottom of the list.

[dlsterner]

I'll take a stab at Maryland.

I see that some users are including Interstate and US routes in their lowest 10; some are not.  I guess it's open to interpretation, but I will go strictly with signed state routes.

There currently is no MD 1 nor MD 9.  Since there also isn't a MD 11 nor MD 13, the next two lowest are MD 12 and MD 14.

1.  MD 2 - Major N/S route in central Maryland serving Baltimore, Annapolis, and southern Maryland.
2.  MD 5 - Major artery serving Washington DC from Waldorf and southern Maryland.
3.  MD 4 - Similar to MD 5 but a tad less significant.
4.  MD 3 - Although short, part of the major southbound route bypassing Washington to the south.  Carries significant traffic.
5.  MD 10 - Bypasses Glen Burnie fron I-695 to Annapolis.  Was originally part of the proposed Baltimore-Annapolis expressway.
6.  MD 6 - Meanders around La Plata but doesn't connect much of anything.
7.  MD 12 - Connects Salisbury and Snow Hill.
8.  MD 14 - Meanders around the Eastern Shore; doesn't serve any cities or towns of note.
9.  MD 7 - Piecemeal route that is largely the former US 40, which mostly replaced it.
10.  MD 8 - Short road serving residents of Kent Island.  Except for the residents, not terribly useful.

I tried to go by usefulness and not just by length.

Comments:
Using historical routings of MD 3 (when it was longer), it might have been higher on the list.
If the MD 10 corridor ended up as the Baltimore-Annapolis expressway rather than the I-97 routing, it might have been higher.

If I were to include US and Interstate highways, US 1 and US 11 would replace MD 12 and MD 14.  I suspect US 1 would go to the top of the list, and US 11 would beat out MD 8 and MD 7, but not much else (by being short and by being largely replaced by I-81)

Just my opinion; your opinion may vary.

[US20IL64]

IL-1 is the longest state route, so that has to come first.
IL-9 runs from Iowa to Indiana, spanning the entire state, serving as the main east-west route through Bloomington.
IL-3 is the westernmost state highway along the Mississippi River from just north of the Metro-East all the way to Cairo.
IL-4 is a long state route from Springfield along parts of US66 and continues south of the Metro-East.
IL-2 follows the Rock River from Sterling through Rockford to the Wisconsin border.  I'd say it's important.
IL-10 is semi-long from Champaign through Clinton and Lincoln to somewhere east of Havana.
IL-7 is pretty short but it heads through the suburbs, garnering more of an ADT.
IL-8 is pretty short, going through Peoria.
IL-6 is a freeway, but it's short and not very heavily traveled.
IL-5 is a small surface road fragment left behind by the christening of I-88.

I admittedly mainly based this on the routes' length and not much else.

There is an interchange on I-88 called "Former IL 2", south of Sterling area, so it used to go further. And, yeah IL-5 was the whole route of I-88, from around 1971 to '87.

[DandyDan]

For Iowa:
1. US 6- Much of it is overlapped with interstates, but the parts that are not include major surface roads in Des Moines, Iowa City and Davenport.
2.-4. IA 3, IA 2, IA 9- All three are border-to-border routes, or nearly so. I put 3 first because it gets to the Dubuque area and 2 second because of its short expressway section that leads to Nebraska.
5. IA 5- Part of it is the Des Moines southeast bypass and presumably the route people use to go to the Des Moines airport. Outside the Des Moines area, there is an expressway section and it's the road to Rathbun Lake.
6. IA 1- A major road in Iowa City and also Washington and Fairfield
7. IA 4- Somewhat lengthy, but the largest city on it is Estherville.
8. IA 7- A shorter route, but it does lead to Fort Dodge.
9. IA 10- Similar to IA 7, but its largest city is Orange City.
10. IA 8- A short route east out of Traer through Dysart to US 218 on the way towards Vinton.

[skluth]

Missouri - All routes are state routes

7 - Has an important four-lane expressway segment from Harrisonville to Clinton
5 - Longest route in the state and has a tiny freeway segment near Lake of the Ozarks
9 - Only 15 miles but a busy corridor in KC
1 - Even shorter, but another busy corridor on the KC North Side
6 - Second longest low-numbered state route in MO
10 - Has a busy commuter exit from I-35 into Excelsior Springs, but otherwise nondescript
2, 3, 8, 11 - Don't really care at this point. All are pretty irrelevant.

4 no longer exists

[JayhawkCO]

Colorado:
U.S. 6 - its significance is diluted by a long concurrency with, or being a parallel surface route to I-70 over much of its length, but it is an independent route carrying traffic to Nebraska from the northeast plains.
CO-9 - Longest single-digit state highway, through the eastern mountains from U.S. 50 near Canon City to U.S. 40 at Kremmling.
CO-10 - Connects Walsenburg and La Junta. From the map, it looks like it should be busier than it is.
CO-7 - Zigzags across the plains-mountain interface north of Denver to Estes Park.
CO-5 - The road up Mt. Evans
CO-8 - Keeping the old concurrent state number for U.S. 285, a short state highway across the hogback on the west side of Denver.
CO-2 - Colorado Blvd. in Denver
CO-1 - Keeping the old concurrent state number for U.S. 87, a short state highway connecting I-25 and U.S. 287 north of Ft. Collins.
CO-3 - A short state highway in Durango.
CO-11 - A short state highway connecting far northeast plains towns to a county road in Nebraska.

No CO-4.
If you eliminated U.S. 6, I would put CO-12, the scenic loop from Trinidad west and north over Cucharas Pass to U.S. 160 west of Walsenburg above 5 and below 7 (i.e., 4th position).

I know this one was done 6 six years ago, but I'd arrange as follows:

US6 - Just the highway segment in the Denver metro area alone makes it most importnat
CO2 - Colorado Boulevard is one of the busiest stretches of state highway in the state
CO7 - An important regional route in the northern exurbs of Denver and a popular route to Estes Park
CO9 - Very heavily traveled between Frisco and Breckenridge and the quickest way up to Steamboat
CO10 - The long regional route between Walsenburg and La Junta connects two of the most important junctions in SE CO
CO8 - Important to get to Red Rocks
CO5 - Mt. Evans road is awesome, but not important
CO1 - Pretty minor road that feels like it should be a city street, not state highway
CO11 - At least it goes somewhere (Nebraska)
CO3 - Goes nowhere and is barely signed

Chris

[US 89]

I'll redo Utah:

I-15 - well, duh. Easily the most important route in the entire state.
US 6 - major cross-state route, especially the portion between I-15 and I-70 which carries significant interregional traffic volumes
SR 9 - gateway to Zion National Park and a fairly major 4-lane expressway in the northern St George areas
SR 7 - St George's beltway route, mostly a super-2 but full freeway from I-15 to almost the airport. Would be higher but it currently serves mostly nothing
SR 18 - pretty significant arterial in St George, and also a decent alternate route to I-15 over Black Ridge
SR 12 - easily the most scenic route in the state and probably close to that spot for the whole US. Links Bryce Canyon, Grand Staircase, and Capitol Reef
SR 10 - essentially Emery County's Main Street, links about 10,000 people to I-70 and US 6
SR 17 - the road connecting Zion to the north
SR 14 - one of a few cross-mountain routes connecting I-15 and US 89, is decently scenic but not exactly friendly during winter or anytime for trucks
SR 16 - connects the few hundred people in the boring part of Rich County to Wyoming and other roads to better parts of the county
SR 13 - old US 30S/191 through northern Utah, has now been replaced by I-15 but is still a valid surface alternate
SR 8 - part of old US 91 in St George, but very short and really should either be extended through the rest of old 91 or dropped entirely

Edited to note: there is no SR 1 through 5 because those numbers used to be the unsigned legal designations for Utah's five interstates, and they were never reassigned after internal designations for interstates and US highways were eliminated in 1977. There is also no SR 11, which was the legal and signed designation for the short Utah portion of US 89A before 2008.

[Max Rockatansky]

Florida

1:  FL 8 (hidden designation for I-10)
2.  FL 5 (hidden designation for much of US 1
3.  FL 10 (hidden designation for much of US 90)
4.  FL 7 (hidden designation for much of US 441 in the Miami-Dade Area)
5.  FL A1A (has length, scenery and brings a lot of tourism)
6.  FL 2 (mostly based off length
7.  FL 9 since it is a surface highway in the Miami Dade Area
8.  FL 6 given it has decent rural length.
9.  FL 4 given it is similar to FL 6 but shorter.
10.  FL 3 since it has a short length mostly connecting military facilties.

With Arizona obviously it is I-10 followed by I-8.  Nothing doing in this thread for the entire State of Nevada unless you want to do pre-1976 numbers.

[Max Rockatansky]

California:
I-5
I-10
CA-1
I-8
CA-4
CA-3
CA-2
CA-9
US-6
CA-7

I could argue about the order of 4-3-2-9, but I give 4 and 3 the nod because of their length.

Taking my own stab at modern California:

1.  I-5
2.  I-10
3.  CA 1
4.  I-8
5.  CA 4 (way too much freeway and a Trans-Sierra Pass not to be in the top five)
6.  CA 2
7.  US 6
8.  CA 9
9.  CA 7 (a minor border crossing)
10.  CA 3 (a long highway but one in a corridor that hasn't been relevant since the California Gold Rush)

1934 Version

1.  CA 7 (founded the basis modern of I-405, US 6, modern CA 14 and US 395)
2.  CA 1 (might contend for spot 1 if it was built through the Lost Coast)
3.  CA 3 (became US 101A and is highly known for modern CA 1 through Malibu)
4.  CA 4 (Ebbetts Pass would have been way more important in 1934 and it still had some big time length)
5.  CA 9 (Big Basin State Park and way longer length than the modern highway)
6.  CA 8 (another Trans-Sierra Pass but not as long as CA 4)
7.  CA 2 (Wasn't built fast enough over Angeles Crest and the dirt segment over modern CA 173 was incredibly wonky)
8.  CA 5 (Skyline Boulevard sure is pretty)
9.   CA 10 (Major street in the Los Angeles Area but nothing greater in importance compared to the above)
10.  CA 6 (was renumbered to CA 26 relatively quickly to make way for US 6)

[CoreySamson]

Texas doesn't have an official state route 1 and 2, but we do have NASA 1 and plenty of loops, spurs, and FM roads to choose from for 2. Here's my hierarchy:

TX 6
TX 7
TX 3
TX 10
TX 4
NASA Road 1
TX 5
Loop 2
TX 9
TX 8

[Scott5114]

Unlike a lot of states, Oklahoma has a full suite of one-digit highways, all of them are fairly substantial.

SH-9: Not only is it a major expressway in the state's third-largest city, it's a border-to-border route linking southwest Oklahoma with the Arkansas border. After crossing into Arkansas, it immediately becomes an Interstate. Call me a homer, but this one is probably at the top of the chart.

SH-3: The longest state highway in Oklahoma, and one of the longest in the nation. But more than half of it is tacked on to US highways, so should that really count toward it? What's left is still a fairly major route that connects OKC with southeast OK, so it still deserves a high ranking. Just maybe not as high as the book length alone would suggest.

SH-1: SH-1 didn't get its designation because it's an important road, but because it was judged the most scenic. It's hard to disagree with that assessment (unless you're a SH-325 fan). That means that it's known about far more than its length and places it serves would suggest.

SH-4: A major arterial in OKC's west surburbs, before swinging down to the southwest exurbs and finally finishing with a new extension onto an 80 MPH turnpike, which did a lot to bump it up this high. There's also the other SH-4, which is a short connector to Arkansas, but that route doesn't really add a whole lot to SH-4.

SH-7: The backbone of south-central Oklahoma. Lawton to Davis and Sulphur to Atoka. It's the only way to get from I-35 to the Chickasaw National Recreation Area (formerly Oklahoma's only national park).

SH-10: This one is a little odd; an L-shaped route that basically mirrors Oklahoma's northern and eastern borders up in the northeast corner of the state. It's certainly got the length (233 miles) but only serves three county seats along the way (Miami, Grove, and Tahlequah). Those brown Scenic 10 shields are pretty neato, though.

SH-8: Not a glamorous route, but a workhorse. As part of its concurrency with US-281, it's part of the main route to northwest Oklahoma, as well as over the old US-66 Canadian River bridge (the one with a million pony trusses).

SH-2: Makes sense that the highway with the number 2 has two parts. But unlike 4, this one used to be connected until US highways got in the way. That said, what's left is a not terribly important north-south route that only serves five counties, entirely missing the seat of two of them. At least it's decently scenic (it overlaps SH-1 for a little while near SH-1's most scenic section). One of its termini are while it's concurrent with US-266, which also ends there, so this route gets photographed way more than it probably should.

SH-6: A C-shaped route that kind of rambles around southwestern Oklahoma, though it manages to hit two biggish cities, Elk City and Altus, so it's not like it's useless or anything.

SH-5: Another twofer. The western section is just a short bit of road linking two other highways, while the east section takes a circuitous route through three minor county seats. Far from the least important highway in the system, but definitely the least important one-digit route.

[Road Hog]

Arkansas is stingy with 1-10 numbers. The numbers 2, 3 and 6 are unused and 4 barely exists. That leaves AR 11, AR 12 and AR 13 as the next lowest numbers to make 10. I'll rank them:

AR 7: Longest state highway in the state and the most scenic by far. Draws tons of tourists. Serves the cities of Harrison, Russellville, Hot Springs, Arkadelphia and El Dorado.

AR 10: Important in that it serves the Little Rock area as a major thoroughfare (Cantrell Road). Otherwise, it's pretty rural as it connects the Fort Smith area with Little Rock in a straight shot, but is 55 mph and crawling with local yokels in every little town.

AR 5: Also serves the Little Rock area, but mostly suburban/exurban parts. Discontinuous, as it ends at Future I-57 in Cabot and picks up again on the southwest side of Little Rock, parallels I-30 to Benton and then bounces west to end at AR 7 in Fountain Lake. Outside of the metro, it is very rural.

AR 12: Truncated in the 1930s when U.S. 62 was created, but today serves rapidly-growing NWA.

AR 1: The most important north-south state highway in the Delta, but it has been largely subsumed by US 49 and the population in the Delta has cratered.

AR 9: Discontinuous with a weird gap in the Malvern area. Almost completely rural with some scenic stretches. Malvern and Morrilton are the two largest towns it serves.

AR 8: Almost completely rural as it crosses southern Arkansas at a diagonal. If you like pine trees, AR 8 is your huckleberry.

AR 13: Unimportant farm road otherwise that was recently extended to become a western loop around Searcy.

AR 4: Used to cross south Arkansas but was converted to US 278 in the 1990s, and has only two short segments left on either end of the state.

AR 11: Three discontinuous segments in the flatlands.

(Note: AR 2 is now entirely US 82; AR 3 is now US 79 between Pine Bluff and West Memphis with other segments renumbered; and AR 6 is now entirely US 270 with a separate segment in eastern Arkansas from Brinkley to Helena transferred to US 49. That's what happened to them.)

[SkyPesos]

Rankings for the first ten 1922 MO state routes and 1923 OH state routes. Both state gave the most important routes the lowest numbers

Missouri:
MO 2 - Cross-state route connecting the two largest cities in the state.
MO 1 - Cross-state route along the western edge of the state, via Kansas City. Placed this one a hair above MO 9 as the other cities on MO 1 (St Joseph, Joplin, etc) are larger than other cities on MO 9 (Hannibal, Cape Girardeau, etc).
MO 9 - Cross-state route along the eastern edge of the state, via St Louis.
MO 7 - Cross-state route along the center of the state, passing through the state capital, and the city of the state flagship university.
MO 8 - Cross-state route along the north central part of the state, connecting St Joseph and Hannibal.
MO 3 - Cross-state route passing through Springfield, Branson and Sedalia.
MO 10 - Links Kansas City to Hannibal, though is shorter than the other E-W routes listed here as it ends at MO 8 at Monroe City.
MO 5 - I guess it's most rural nature back then is why it still exists in roughly the same length today.
MO 6 - Besides St Joseph, this route is mostly rural.
MO 4 - Cross-state route along the northern edge of the state that doesn't pass through any significant cities.

Ohio:
OH 3 - Connects the 3C cities.
OH 6 - Connects Cincinnati, Dayton and Toledo.
OH 2 - Cross-state route serving Toledo and Cleveland.
OH 7 - Longest state route in Ohio, running along the entire southern and eastern edges of the state between Cincinnati and Conneaut.
OH 1 - Follows the National Road.
OH 8 - Goes between Cleveland and Marietta via Canton and Akron.
OH 4 - Portsmouth to Sandusky route via Columbus
OH 5 - Follows the Lincoln Highway. Passes through mostly smaller cities than the routes above.
OH 9 - Paralleled the more important OH 6 to the west, passing through mostly rural areas north of Cincinnati.
OH 10 - Acted like a southern "loop" route for OH 5, via Lima and Marion.

[paulthemapguy]

Rankings for the first ten 1922 MO state routes and 1923 OH state routes. Both state gave the most important routes the lowest numbers

Ohio:
OH 3 - Connects the 3C cities.
OH 6 - Connects Cincinnati, Dayton and Toledo.
OH 2 - Cross-state route serving Toledo and Cleveland.
OH 7 - Longest state route in Ohio, running along the entire southern and eastern edges of the state between Cincinnati and Conneaut.
OH 1 - Follows the National Road.
OH 8 - Goes between Cleveland and Marietta via Canton and Akron.
OH 4 - Portsmouth to Sandusky route via Columbus
OH 5 - Follows the Lincoln Highway. Passes through mostly smaller cities than the routes above.
OH 9 - Paralleled the more important OH 6 to the west, passing through mostly rural areas north of Cincinnati.
OH 10 - Acted like a southern "loop" route for OH 5, via Lima and Marion.

I thought Ohio 1 was the "three C's" route that largely was replaced by I-71? Or did that come later than 1923?

[SkyPesos]

Rankings for the first ten 1922 MO state routes and 1923 OH state routes. Both state gave the most important routes the lowest numbers

Ohio:
OH 3 - Connects the 3C cities.
OH 6 - Connects Cincinnati, Dayton and Toledo.
OH 2 - Cross-state route serving Toledo and Cleveland.
OH 7 - Longest state route in Ohio, running along the entire southern and eastern edges of the state between Cincinnati and Conneaut.
OH 1 - Follows the National Road.
OH 8 - Goes between Cleveland and Marietta via Canton and Akron.
OH 4 - Portsmouth to Sandusky route via Columbus
OH 5 - Follows the Lincoln Highway. Passes through mostly smaller cities than the routes above.
OH 9 - Paralleled the more important OH 6 to the west, passing through mostly rural areas north of Cincinnati.
OH 10 - Acted like a southern "loop" route for OH 5, via Lima and Marion.

I thought Ohio 1 was the "three C's" route that largely was replaced by I-71? Or did that come later than 1923?

That came later, in 1961, as the second iteration of the number.

Fun fact: OH 6 was used on 3 different state routes before US 6 was extended into the state. First version roughly on current US 25, second version on US 250, and third version (much shorter than the previous two) on OH 283.

[JayhawkCO]

I'll do Wyoming since no one else has. No single digits to be found.

US14 - Eastern entrance to Yellowstone and a major thoroughfare across the north of the state
US20 - Albeit multi-plexed for most of its length in Wyoming, connects Yellowstone, Worland, and Casper and runs through the middle of the state
US16 - Another one that's multi-plexed a lot, but runs either together or parallel to US14 and connects to Rapid City
WY22 - The only state highway worth much on this list.  Connects from Jackon to Victor, ID over Teton Pass
WY24 - Gets you to Devil's Tower and heads a back way to Belle Fourche/Spearfish
US18 - Doesn't do much in Wyoming, but runs east from I-25 into South Dakota, eventually making its way to Milwaukee
WT13 - Connects I-80 to the barren part of US30
WY11 - Connects the very small town of Albany to the outside world. 
WY10 - Runs from WY230 to the Colorado border in the very southern part of the state.  Provides access to the Jelm Mountain Observatory
WY12 - Basically a frontage road to I-80 near Laramie

Chris

[FrCorySticha]

Montana had a full suite of single-digit highways, though many have disappeared over the years. So the first 10 MT state highways (no US or Interstate):

MT 3 - Mostly duplexed route (with US 12, US 191, US 87 and US 89) between Billings and Great Falls, serves as most direct route between two important cities
MT 16 - Farthest east N-S route, important for Billings-Regina SK traffic
MT 7 - N-S route that provides connection between I-94 and Ekalaka in the extreme Southeast part of the state
MT 13 - N-S route in Eastern part of the state between Circle and Canadian border
MT 1 - Scenic drive between I-90 at Drummond and I-90 at Anaconda (Former US 10 ALT)
MT 19 - Connection between US 87 and US 191 in Central part of the state, I regularly used between Malta and Billings
MT 5 - E-W route in far Northeast part of the state, part of MT/ND 5 multi-state route
MT 2 - Alternate route from I-90 between Butte and Three Forks (Former US 10)
MT 21 - Connection between MT 200 and US 287 at Augusta
MT 17 - Short route along east side of Glacier National Park towards Canada and Waterton Park

[Konza]

I'll add to the Illinois posts with some history:

IL 1:  Most of it is still IL 1.  North-south route up the eastern side of the state.
IL 2:  Originally Cairo to South Beloit.  Most of the original IL 2 is now US 51 and I-39.
IL 3:  North-south route up the western part of the state.  North of East St. Louis, most of it is now US 67.
IL 4:  East St. Louis to Chicago route.  First US 66, most of it is now I-55.
IL 5:  Chicago to Dubuque.  Now US 20.
IL 6:  Chicago to Clinton, Iowa, straight west.  US 30 and Alternate 30.  Alternate 30 became Illinois 38 in the early '70's.
IL 7:  Chicago to Rock Island.  Eastern end is still IL 7; the rest is US 6.
IL 8:  Went across the state east-west; most of it is now US 24.
IL 9:  Another east-west route; most of it remains IL 9.
IL 10:  See IL 9 above.  Different route, but most of it remains IL 10.
IL 11:  East St. Louis to Terre Haute, Indiana.  First US 40, now I-70.
IL 12:  East St. Louis to Vincennes, Indiana.  Now US 50.
IL 13:  East St. Louis to Old Shawneetown on the Ohio River.  Still IL 13 although the western part was once US 460.
IL 14:  DuQuoin to Carmi on the Wabash River.  Was US 460 but reverted to IL-14 when US 460 was decommissioned in Illinois.
IL 15:  East St. Louis to Mt. Carmel.  Most of it remains IL-15; parts of Il-15 in Metro East were also US 40 at one time.

[ilpt4u]

I'll add to the Illinois posts with some history:

IL 1:  Most of it is still IL 1.  North-south route up the eastern side of the state.
IL 2:  Originally Cairo to South Beloit.  Most of the original IL 2 is now US 51 and I-39.
IL 3:  North-south route up the western part of the state.  North of East St. Louis, most of it is now US 67.
IL 4:  East St. Louis to Chicago route.  First US 66, most of it is now I-55.
IL 5:  Chicago to Dubuque.  Now US 20.
IL 6:  Chicago to Clinton, Iowa, straight west.  US 30 and Alternate 30.  Alternate 30 became Illinois 38 in the early '70's.
IL 7:  Chicago to Rock Island.  Eastern end is still IL 7; the rest is US 6.
IL 8:  Went across the state east-west; most of it is now US 24.
IL 9:  Another east-west route; most of it remains IL 9.
IL 10:  See IL 9 above.  Different route, but most of it remains IL 10.
IL 11:  East St. Louis to Terre Haute, Indiana.  First US 40, now I-70.
IL 12:  East St. Louis to Vincennes, Indiana.  Now US 50.
IL 13:  East St. Louis to Old Shawneetown on the Ohio River.  Still IL 13 although the western part was once US 460.
IL 14:  DuQuoin to Carmi on the Wabash River.  Was US 460 but reverted to IL-14 when US 460 was decommissioned in Illinois.
IL 15:  East St. Louis to Mt. Carmel.  Most of it remains IL-15; parts of Il-15 in Metro East were also US 40 at one time.

IL 1: Also, from the Chicago South Suburbs to Danville, is also the old Dixie Highway route

[fillup420]

I'm suprised nobody ever did North Carolina, where the low-numbered routes are fairly minor.  Here's the listing in numeric order.

US-1 
NC-2  NC-5/Pinehurst -to- US-1/Southern Pines (5.5 miles, all four-lane)
NC-3  US-601/Kannapolis -to- NC-152/Mooresville (27.0 miles)(designated in 2002 to commemorate Dale Earnhardt, Sr.)   
NC-4  US-301/Battleboro -to- US-158/Littleton (34.7 miles)
NC-5  NC-211/Pinehurst -to- Malcolm Blue Farmstead park entrance/Aberdeen (8.3 miles)
NC-6  I-40/Greensboro/Patterson Street/Exit 216 -to- I-40/Greensboro/Lee Street/Exit 224 (7.9 miles)(decommissioned 2004)
NC-7  US-29/US-74/Gastonia -to- US-321/Belmont (12.1 miles)
NC-8  US-52/New London -to- VA-8/Virginia State Line (94.6 miles)
NC-9  Montreat -to- SC-9/South Carolina Line (46.5 miles)
NC-10  I-40/Catawba/Oxford School Road/Exit 138 -to- NC-226/Polkville (48.3 miles)
NC-11  US-74/US-76/Freeman -to- US-158/Murfreesboro (193.2 miles)
NC-12  US-70/Sea Level -to- Corolla (148.0 miles)

First place is a no-brainer.  NC-12 runs the entire length of the Outer Banks, plus the entire length of Ocracoke Island, and incorporates the two popular ferries.

Third place is a toss-up between NC-2 and NC-10.  The former is the charming gateway to both Pinehurst and Southern Pines.  The latter used to be the main gateway to Claremont and Newton from points east.

NC-10 was once the Central Highway, which spanned nearly the entire state. Its too bad it was all chopped up by US 19, 64, and 70.