NFL:
Vikings
I will cheer for lots of other teams though: Steelers, Dolphins, Falcons, etc.
MLB:
Braves
I don't watch baseball at all though.
NBA:
Sonics
I don't really watch the NBA though
College sports:
Georgia Tech - school I attend
NC State - school I grew up loving
College sports and the NFL are the only things I really feel strongly about.
We should do a list of teams you hate next, mine would be about a mile long... and I have a legitmate reason for each