I haven't played any of the third gen systems yet, so I can't really judge those for myself. I've always been more of a Nintendo fan because I like the games for it. The PS2 is a better system than the Gamecube though. I never really liked most (not all) of Xbox's selection of games, so no matter how good the performance was, it wasn't a very useful system for me to bother buying.
So basically, if judging by the games, I'd probably say Gamecube, if judging by the system itself, I'd probably say PS2.