Please don't do this...A flame war will ensue.
Pick whichever has the games you want.
The PS3 and X360 are the two systems that will have the most main games since the Wii isn't as powerful and it will be suited to more gimicky games in my opinion to make use of the motion, which I don't personally care for that much.
Deciding between the two of those will be picking which exclusive games you like. Assuming you are a GT player, that gives a reason for the PS3. I can't really think of many big X360 exclusives right now besides Bioshock and Halo 3, I'm not sure what is coming up this year.