As someone who owns all three consoles.
Here's a breakdown of my game collection since owning all 3:
PS2 Games: Manhunt, SD 4, Kingdom Hearts, Virtua Fighter Evolution, Virtua Fighter 4, Pride, Deus Ex, Half Life (Total: 8) (Exclusives: 8)
Gamecube Games: Monkey Ball, Monkey Ball 2, Eternal Darkness, Wind Waker, Metroid Prime, Resident Evil, Resident Evil 0(Total: 7) (Exclusives: 7)
XBox Games: Buffy, Buffy 2, GTA 3, GTA VC, KOTOR, Halo, Splinter Cell, Rainbow Six 3, Crimson Skies, Morrowind GOTY, Midtown Madness 3, Midnight Club 2, Project Gotham Racing 2, Midway Arcade Treasures, ESPN Football, DOA 3, Wolverine, Scooby Doo(yeah...that's right), Evil Dead, Blood Omen 2, House of Dead 3, Deus Ex 2 (today, woo!) (Total: 22)
(Exclusives: 11)
Since I play online a great deal...Xbox is the best for me. On top of that, virtually every game I'm interested in next year comes out for Xbox.