Well, if you are talking about Raw power, it's the PS2 that's going to come up short. And from what I am seeing, the Xbox has quite a nice US showing, but no other country has embraced it yet (And very few people will continue to develop for a system that has a lower installed base)
The Gamecube is Number 2 worldwide, and as an owner of all 3, it is the system that I am keeping a close watch on.
The gamecube will have many of the ports of the games from other systems (probably last in the order) but it will have them most, and the best first party games out there. (Square is the best, but they only release 1 or 2 a year)
Between Star Wars, Resident Evil (Best looking game graphics on any system... Ever.), Pikmin, Super Smash Brothers Melee, and Monkey Ball (Great game, which gets no recognition.) I have a full plate with my Cube.
Soon I'll be adding Mario, Zelda, Metroid, Tales of Phantasia, F-Zero, Donkey Kong, Kirby, MarioKart, and hopefully the Mother series (Earthbound) And all of those are must haves.
For My Xbox.... Well, I like Rallisport, but it doesn't have the longevity (and is way hard) to be a Must have.
On the PS2... GT3, FFX, Tekken, Time Crisis 2 are the only must haves I can think of. (GTA3 is coming for PC and Xbox so it doesn't count as exclusive)
The GC might not get as many games as some of the other systems, but there are so many more good games to be had on the system thanks to Nintendo's Standards of Quality.
My only actualy complaint about the GC is that the Memory card is too small. (both physically, and the fact it only holds 4 megs - but it is cheaper)