Opinion only... take with grain of salt...
If you come from a PC gaming background, the XBOX would probably have the most games that are similar. As Steve mentioned FPS and RPGs.
The PS3 is actually a great way to get a cheap blueray player. It also, IMO, has the most games --
you should have no problem finding a great game for your favorite genre (as a side note, it really bothers me that the newer PS3 units are no longer compatible with PS2 games, WTF).
As for the Wii, this is a must have if you are a Nintendo fan. For example, you are not going to find a Zelda game anywhere else. This system is must for people who grew up with Nintendo and wants the next iterations of games from Nintendo. It is also a great
unit for younger children.
Henry