I have to go with the Xbox. It was the only 6th gen console I played and owned. I just wished I was a Nintendo fan back then 'cause there are several Gamecube games I would love to play: Metroid Prime 1 &2, Baten Kaitos 1 & Origins, Fire Emblem Path of Radiance, Eternal Darkness, Windwaker, Paper Mario Thousand Years door and so on.
But Xbox had also awesome games:
Halo 1 & 2, Kotor 1 & 2, Splinter Cell trilogy, GTA San andreas, Jade Empire, Morrowind, Sid Meier's Pirates, FIFA 2003 and Prince of Persia: Sands of Time.