Depends on what kinda games you like, whether online is important to you and if you want hi def movies and shit.
I'll try to lay it out bullet style:
360
-biggest game library right now
-specializes in sports and shooter titles
-Xbox Live Arcade library is pretty good
-backward compatibility with all of these
XBox games
-XBox Live Gold membership ($50/year I think) makes online play uniform across all games
Wii
-cheapest system (at the cost of graphical power and hard drive space tho)
-big library of "casual gamer" titles (Cooking Mama, Brain Age etc)
-Nintendo titles (Metroid, Zelda, Mario Kart, Smash Bros, etc)
-ssssssexy library of 8-bit, 16-bit and 64-bit era titles (SNES, TurboGraphics, Genesis, N64, NES)
-various channels that provide weather, worldwide and local news, voting polls, Nintendo DS demos, etc
-backward compatible with Gamecube games
PS3
-large hard drives
-backward compatible with PS2 and PS1
-relatively cheap Blu-Ray player
-lots of good upcoming games over the next year (Metal Gear Solid 4, SOCOM 3, Final Fantasy 13, Final Fantasy Versus 13, etc)
-the system to have for RPG titles
-most powerful system (at least on paper)
-10 year life cycle (at least according to Sony)