I pretty much only follow US sports, so my teams are all from the states. These are my official favorites, teams that I would not ever root against unless they play someone else on the list. There are also a ton of teams/individuals I root for casually but aren't entrenched as an "official favorite".
NFL: 49ers, distant second is Steelers
NBA: Kings
MLB: Giants, very very close second is A's
NHL: what's this? Sharks, I guess.
College Sports: Div 1AA - UCD, Div 1A- UCLA
Some individual favorites that aren't official favorites: Woods, Mickelson, Sorenstam, Penske, Mears, Earnhardt Jr, Williams, Williams, Ivey, Chan
Why? I pretty much followed most of the local teams that my parents followed when I grew up. I don't know why I picked UCLA, but they've always been my favorite. I think I tend to like teams with gold in their uniform. UCD because I went there. The Steelers was weird, I always kind of liked them, but when I noticed I was kind of rooting for them year after year, I eventually decided to make it official, so now there is no going back. Most of the individuals I started watching early in their career or when I first started watching the "sport", so I claim it is not my fault that I picked a bunch of dominators.
What about you all?