Xbox has the best specs and the most potential for the typical gamer - on average, any non-bias person has to agree, their games are better at this stage. There's not one horrible Xbox title yet. PS2 has the largest library of titles, the quality of which has never been incredible, but the sheer number sorta makes up for it, I suppose. Gamecube is awesome for little kids, but it's still just that - a console for kids. And Dreamcast, which could have been something legendary, died :(
If you're rich, PS2 is your ideal machine. If you're young (or young at heart), Gamecube is for you. If you're the typical guy like me, Xbox is the only way to go.