County roads are generally better for cycling than state routes, state routes better than US routes, US routes better than interstates. However, the roads that are better for cycling are almost always longer and hillier than the major roads, but cyclotourists are not usually after the shortest path between two points anyway.
A lot of road choice depends on personal preference. Some prefer broad shoulders on high-traffic roads, and others prefer no shoulders on low-traffic roads. It's seldom you get both shoulders and low traffic.
I agree that a lot depends on personal preference. It also varies with geographic location and individual road. That said I don't find smaller better in many cases.
Interstates - Can be ridden in large portions of the west. They can be quite pleasant, quite unpleasant, or something in between. I have ridden sections of Interstate that I hated and sections that I really liked. For example I-25 in NE New Mexico was very nice. Good views of the Sangre de Cristo mountains, clean shoulders, gentle grades, and light traffic made for a very pleasant ride. I'd go out of my way to ride there. The section of I-80 on the TA was loud, not scenic, and the shoulders were wide, but somewhat debris strewn. It wasn't completely awful, but we were glad to be off of it. Access roads for interstates can be quite nice. You do have to be careful because sometimes they just fizzle out.
In the continuum from US routes to county roads I find that which works the best depends on where I am. At home (Baltimore area) when just out for a meandering ride I find county roads nice. In more remote and rural areas I less often ride them though preferring somewhat larger roads much of the time. When going longish distances in these more remote parts of the country I'd rather stick to the fairly direct route and minimal need for complex directions provided by US or state routes. I do like to ride some sections on smaller roads for variety.
In the Ozarks and Appalachians I enjoyed the tiny roads. In the Cascades, the Rockies, and the plains I mostly liked US and state routes. In the Sierras I enjoyed a mix with a bias toward state routes.
BTW, I have a low tolerance for roads in poor condition and avoid dirt or gravel roads if at all possible. If I want to ride off pavement I'll do a MTB tour and try to go all dirt, otherwise I prefer well surfaced roads.