Some of my faves (OO):
YYJ (we stay up in Sidney near the airport)
Guanajuato, MX -- gorgeous town
Bozeman and Missoula MT (in the summer, of course) -- great college towns
Wichita -- downtown was a pleasant surprise
And then of course the obvious ones: SBA, SBP, MRY, EUG, SUN (in the summer), SMF (we stay right downtown).
On the flip side, I'll be just fine if I never see the following places ever again: Williston, Winnipeg, Fresno, Charleston WV, Edmonton, Idaho Falls, and let's not forget Devil's Lake :-0