Most important "state route nnn" from all 50 US states

[Kacie Jane]

It will vie with Florida, Virginia, and Pennsylvania for the numbers above 500, since few states bother with them.

Not Virginia, unless you're including its secondary state routes 600 and higher.

But Maryland has quite a few state routes in the 500-999 range, perhaps including some semi-important ones (MD 528, main drag through Ocean City, comes to mind).

Ah, that was the other one I forgot with multiple "high numbers".

Washington has several 5xx's (spurs off I-5), I believe two 7xx's (spurs off SR 7) and a small handful of 9xx's (spurs off I-90).  And IIRC, there used to be both a 601 and a 801, but neither exist anymore.

The 700s and 900s are nothing exciting (though perhaps only as unexciting as the high-numbered routes in other states), but many of the 500s are vital links in the Puget Sound area.

[corco]

704 is going to be the Cross Base highway in Tacoma, so it will be pretty important. 970 is a key link for Seattle to Wenatchee traffic.

[Kacie Jane]

Gah!  I knew there was something after 90x, but I totally blanked.  Also forgot the 82 spurs.

[Alps]

New Jersey's important 3-digit state routes are 139 (already mentioned), 208, and 440 (not yet mentioned). 444, 445, 446, and 700 all count (not yet mentioned either) if you allow unsigned designations. Ones that aren't unimportant but probably fall to other states: 124 (not yet mentioned), 138 (already mentioned).