Well, the PS2 version is has been out for a while, and you can probably get it fairly cheap, but the Wii and 360 versions have a lot of new stuff that version doesn't have. Between those two, I have to say that the Wii control scheme turns me off a bit, not quite sure how it'd work, so I'd go with the 360 version (unless you're kinda broke, then go PS2).
Talking about how fast you beat a game is like talking about how fast you are in bed. - PredatorX_x
According to this article: http://www.gamer20.com/gamehub/bully-scholarship-edition-x360/previews/804/ the Wii controls are nothing more than "functional", and the 360 is running on a different engine to the PS2 and Wii versions, with visibly better graphics, while the Wii graphics are more similar to the PS2 version.