Well now then.
I own an Xbox, a PS2 and a Gamecube. What a greedy git eh?
I have to say that I play most on the Xbox, simply because I think the games look, sound and play the best. It seems a lot more adult than the other two, especially the Gamecube. Not only that but you don't need a memory card for Xbox, it has a hard-drive. Also Xbox has Xbox Live, which is the most fun I've had playing games in years - and they were just the demo games! Being able to blow someone up spectacularly whilst baiting them through the headset is great. Mechassault is a must if you have an Xbox with Live.
I'm selling my PS2, might sell my gamecube, but my Xbox is there for keeps.
Oh and if you Xbox is chipped you can run game emulators which will play old arcade machines, Nintendo64 games, Amiga, Spectrum, NES, SNES, Megadrive, C64 games. So all those old game systems you had, you can have them again like a little VG library.
Game once had Fifa 2003 running on all 3 machines side by side. PS2 looked the worst. Gamecube looked a bit blocky but better than PS2. And Xbox looked so smooth. The resolution is very good.
I can't recommend the Xbox enough. My girlfriend is getting me Starwars: Knights of the Old Republic for Christmas - can't wait for Dec 25th