PS3 mainly because I'm more interested in it's game like...
and also some multiplat games.
I support Xbox.
PS3 - Sony's first party efforts.
Wii - for the very same reasons I bought N64 and Gamecube:
ZELDA and MARIO.
Ah! and DS for Nintendo's games and JRPG's.
PS3: the quality of the hardware and the exclusive games... amazing!
Xbox 360: Dead Rising, Perfect Dark Zero, Condemned, and the promise of Halo 3 were the big ones for me.
PS3: Was a graduation present.
Wii: I absolutely love Nintendo, and they never disappoint me. Twilight Princess, Mario Galaxy, Red Steel (which turned out to disappoint), a new Smash Bros game, and the promise of new forms of immersion which are just being realized with games like Silent Hill and The Conduit.
DS: Just felt like buying it on the first day, though the Prime Hunters demo and the promise of a portable Mario 64 helped.
PSP (since sold): Was a present when I was in a bad place in life, but I did really enjoy Twisted Metal: Head-On and was attracted by having a movie player in my pocket.
Brawl. Or I'll still be playing my Gamecube.