Owning all 3 consoles...
I like the Xbox the best. To me, aside from RPGS (Fable is coming out soon though), it has the best selection of games, and features. Graphics are MUCH better than PS2 (Gamecube is a close 2nd to Xbox), Online play is the best.
GCN is a niche console, it has a few good games that has such high quality, its just not available on other consoles, but game selection is very very limited.
PS2 on the other hand, is the best RPG Console out, thats about all it has going for it IMO. Xbox also has Fable coming out soon, so I hope thats good.
I perfer the racing games on Xbox (PGR2 for now... Forza soon), Shooters on Xbox (Halo, soon Halo2, RS3, Riddick, etc), and fighting games for Xbox (DOA3, DOA Ultimate/whatever is soon).
Well... what have you decided on?