Demoman for sure these days. Mainly because I've gotten a lot better at getting direct hits with the pipe-bomb launcher, and Splendid Screen + Persian Pursuader is just darned fun.
After that, it depends, but I tend to play Engy and Heavy the most, sometimes pyro and soldier, rarely the other classes. I used to like sniper a lot but frankly in TF2 its pretty hard on most maps, except for 2Fort, and usually you aren't really helping your team that much. Back in the TFC days there were lots of good maps to play sniper on, 2Fort, Well, and Avanti in particular.
Spy's drive me nuts, especially with the dead ringer.
I do miss many aspects of TFC that are gone from TF2 - grenades, and medic infections in particular.