I have all 3.. I generalize my feelings on each like this..
They all do have a few games worth the console, but:
XBOX - most fun to hack. Halo is a great game but, get over it. It's been a year, and what else to show exclusive-wise? Sneakers? Blinx the time cat? For stuff that hits all 3, the xbox port would be my first pick, but Halo as a selling point is getting tired. It's a good FPS, even a great one, but a hella old one. At this point, its like saying "Buy a PC because it has Quake!" I feel really let down by MS over the last year. There's some good shit on the horizon (Panzer Dragoon, Halo 2, etc) but they seem to think they can just release anything and expect it to sell consoles. I play xSNES more than anything else lately.
PS2 - most games, by far.. Many really good, many really crappy.. DMC is great, Smackdown is the best wrestling franchise on any console, tons of oldschooly goodness like Maximo, Contra, Rygar lately.. They have the developer mindshare in a big way.
Gamecube - highest quality games. I mean, nintendo just polishes the shit out of everything, and/or makes the 3rd party exclusives as polished. They spend much longer on playtesting and QA than the others. This has really always been a key to their success. Mario Sunshine, StarFox, Metroid Prime, Smash Bros Melee, Resident Evil, Eternal Darkness (must have), Rogue Squadron, Pikmin, Animal Crossing, Kirby.. There's (practically) never any noticable quirks or glitches.. Even if you dont like the game, the quality of the game always shines through. No clipping voices, no jerky animations, no half-rendred cutscenes, no glitch cheats..
All this is debatable, big deal. I'm a gamer, not a fanboy. I play games, not consoles.
In the end, I really dont give 2 shits who made which plastic box. Anyone who does always comes off as a bit of a whiny douchebag.