(I’m counting only the actual numbered routes for this)

Kanto: Route 3 is legendarily good and probably the best, but I do have a huge fondness for Route 11.

Johto: Route 42. Routes 38 and 47 are very nice as well.

(I’m not sure whether Route 26 would count toward Kanto or Johto, but I do love that theme too.)

Hoenn: Pretty much a three-way tie between Routes 113, 119, and 120.

Sinnoh: Either Route 209 or Route 225. Honorable mentions for Routes 203, 205 (this is what first pops into my head when I think “route theme,” so clearly it set some kind of example with me), and 216.

Unova: Tough choice here. It’s a close race between Route 10 (cliché answer, I know), Route 11, Route 22, and Route 23. I thiiiiiiiiink... I’m going to say Route 11. I’ve got a lot of nice memories associated with that one.

Kalos: Route 8 is, I think, my favorite route theme from any game. Nothing really gives me a sense of grandeur and adventure quite as profoundly as that one. Routes 15 and 18 are really good too.

Alola: Not a whole lot of options, but I’ll say Route 4 (that is, the Akala Island routes).