It totally depends what sort of thing you're looking for. This is my business, so I know my shit

PS2 - has been out longest and is therefore the biggest by a long way. You'll get the broadest choice of games on this, with anything from arcade stuff to full on Japanese role playing games. Hundreds and hundreds of titles available.

XBOX - Live is their big thing. Online gaming against random teenage yoof from Oregon/Utah etc. You have to pay a subscription for live (about $6 a month) but you don't have to sign up, There's plenty of games to play offline. Great if you like those kinds of games (racing, sports, shooting etc) but a bit limited beyond that. I'd say it's the perfect system for getting friends round. Loads of multiplayer games (so long as you like sports and racing games) and the online thing (Live) which is excellent.

Gamecube - much smaller selection, but a much higher quality of game. Has a reputation as being a kids system, but is anything but.

Depends what you like. If you want racing games, get an Xbox and buy Project Gotham Racing, Toca Racecar or Microsoft Rallisport Challenge 2. All of them are absolutely brilliant. Topspin is the best tennis game you'll find on any system. ESPN NFL or NFL Street are fantastic football games. PLenty of quality MLB/NBA/NHL titles on it too. As mentioned above you can get the Grand Theft Auto double back on Xbox now, which is one of the finest games ever. You're a Miami gangster working your way up the ladder, basically. Fucking violent and very well done. It's the kind of game that shows people why video games are now bigger than Hollywood.

Feel free to PM with any specific questions.