Longest State Route You Haven't Been On.

[AABattery]

For me, it would be VA 48, which is the longest in VA

The longest one I have been on in VA is VA 42 (3rd longest), from Bland to New Castle (yes I know, they're technically two different segments), and the longest one clinched is VA 100 (which is ~27th, and I'm surprised 311 isn't longer but I digress)

Now for each state I've visited (most of these will probably be near the if not the top of the list in their respective states):

Haven't been on (longest)
FL: FL 5
GA: GA 11
KY: KY 7
MA: MA 28
MD: MD 2
ME: ME 11
NC: NC 24
PA: PA 18
SC: SC 9

Haven't been on (2nd place)
CT: CT 8
NJ: NJ 47
NY: NY 5
OH: OH 3
TN: TN 15
VT: VT 30 

Haven't been on (3rd longest)
NH: NH 9

Haven't been on (5th longest)
WV: WV 28

[jlam]

Haven't Been on Longest
AL 3 (374.38 mi)
AR 7 (296.01 mi)
DC 295 (4.29 mi)
GA 11 (376 mi)
ID 75 (170.67 mi)
IA 3 (323.1 mi)
KS 4 (369.08 mi)
MD 2 (79.24 mi)
MN 1 (345.95 mi)
MS 15 (322.77 mi)
MT 200 (706.62 mi)
NV 318 (110.76 mi)
NM 120 (119.03 mi)
ND 200 (415.78 mi)
OR 140 (237.01 mi)
SC 9 (259.57 mi)
SD 34 (419.01 mi)
UT 24 (160.29 mi)
VA 48 (321.2 mi)
WA 20 (436.13 mi)

Haven't Been on 2nd Longest
MO 13 (290.61 mi)
NE 2 (422.29 mi)
OK 9 (348.1 mi)
TN 15 (261.64 mi)
TX 6 (476.38 mi)

Haven't Been on 3rd Longest
CA 299 (305.78 mi)

Haven't Been on 4th Longest
AZ 264 (154.35 mi)
CO 59 (173.3 mi)
FL 10 (394.524 mi)

Haven't Been on 5th Longest
WY 28 (96.5 mi)

Huh. I didn't expect Wyoming to be the best for me.

[pianocello]

It's been five years since I posted here, so I'll check in with some new states:

Florida: SR 30 (11th-longest). Longest standalone: SR 70 (second-longest of those that aren't hidden designations for Interstates or US Routes)
Illinois: IL 127 (12th-longest)
Indiana: SR 9 (5th-longest, for now)
Iowa: IA 9 (2nd-longest)
Michigan: M-25 (7th-longest)
Wisconsin: WIS 27 (5th-longest)

As far as I know, I haven't been on the longest route of any other states, unless there's an overlap I'm not aware of.

[74/171FAN]

PA: PA 414 (77.32 miles)
NY: NY 30 (291.13 miles)
NJ: NJ 70 (60.22 miles)
VA: VA 57 (86.45 miles)
MD: MD 32 (50.63 miles)
NC: NC 27 (197.17 miles)
DE: DE 16 (30.16 miles)  (I am looking to get a full clinch of DE eventually.)
WV: WV 20 (253.65 miles)
OH: OH 4 (209.47 miles)

*Mileage is based on Travel Mapping

I have not driven enough mileage in other states to bother.

Updating since DE is fully clinched, and I have traveled on NY 30, NJ 70 (clinched), MD 32 (clinched), WV 20, and OH 4 since my original post.  I still do not think that any other states are worth doing at the moment besides AZ since I did do a full-on trip there about a year ago.

NY:  NY 3 (244.64 miles)
NJ:  NJ 72 (29.35 miles)
MD: MD 224 (26.71 miles) 
WV:  WV 10 (137.71 miles)
OH: OH 13 (165.09 miles, feels like one of the few north-south routes that I have not snuck in at least a concurrency on)
AZ: AZ 87 (286.67 miles, longest route in the state, was on my original route plan in September 2021, but got cut in the midst of redoing my route plan so that I could close the gap on US 60 between Phoenix and Globe on the Friday of that week as US 60 was closed for a bridge project on Tuesday)

Updating since MD is fully clinched in Travel Mapping now, and a few other states are worth adding IMO:

PA: PA 247 (49.74 miles, I gained mileage on PA 414 back in December 2022 while clinching PA 287)
NJ: NJ 36 (24.65 miles, I am looking to finish NJ soon at this point)
OH: OH 47 (105.80 miles, I was just east of the eastern terminus while clinching US 23 between Toledo and Columbus last month)
FL: FL 70 (150.29 miles)
IN: IN 56 (195.46 miles)
KY: KY 52 (159.86 miles)
GA: GA 7 (217.54 miles)
IL:  IL 16 (175.42 miles)
TN: TN 100 (156.71 miles)
SC: SC 34 (185.22 miles)
AR:  AR 7 (297.15 miles, longest)

I do not think I have done enough travel in other states to add them yet though LA 1, M-28, and MO 5 are the longest and my longest for those states.

Today's update though my list file has not been updated yet to include PA 247.

PA: PA 187 (41.78 miles)
VA:  VA 72 (43.78 miles)

[boilerup25]

NJ: NJ 68 (8 miles)
CA: CA 190 (178 miles)

[Dough4872]

Pennsylvania: PA 18 at 205 miles
New Jersey: NJ 23 at 52.63 miles
Delaware: DE 17 at 8.30 miles
Maryland: MD 4 at 64.85 miles

[74/171FAN]

Delaware: DE 17 at 8.30 miles

I honestly would have expected DE for you to be something a bit longer than DE 17.

[dfilpus]

North Carolina: NC 12, 14th longest.

Since the, I've driven all of NC 12 from Hatteras north. Now the answer is NC 10, 48.3 mi, #54 in state.

[formulanone]

I did a recount in Travel Mapping; was off by a few here and there and had a few new ones to add...

AL: 63 (38th, excluding hidden SRs)
FL: 65 (17th, excluding hidden SRs)
TN: 131 (22nd, excluding hidden SRs)
MS: 341 (36th)
NC: 12 (14th)
TX: 207 (6th)
IN: 66 (12th)
LA: 4 (5th)
PA: 144 (11th)
IL: 16 (8th)

[cenlaroads]

In the two states I've driven in the most:

LA:  LA 308
TX:  TX 16