Faves:
Medic. The presence or absence of medics on a team makes all the difference. This is where I get most of my points, too.
Demo. I hate being dominated by engineers, so I've made a point of getting good at taking out sentry nests.
Hrm... I'd have to pin the title of "Favorite" on Pyro, Engie, and Medic. Pyro because nothing is sweeter than dropping above from a dark corner and toasting seven or eight people from behind, Engie because he's reminiscent of Warcraft and the whole "Where the hell do I put this?!?!" aspect of the game, and Medic because... well... I like helping people