Most fun I've had on an xbox:
Fable
Halo
Halo2
DOA Xtreme Beach volleyball
Fatal frame
Fatal frame 2
Grand theft auto games
And of course all of the emulator roms you can play on these when they're modded (beats playing them on a PC). Playing a translated version of final fantasy 3 (for NES - never released in US) was one of my highlights
Lots of other good games people usually recommend:
Baldur's gate games
Sudeki
Sonic heroes
Project gotham racing 2
Manhunt
KOTOR 1+2
Leisure suit larry (for the old-school pervs out there)
prince of persia
conker live and reloaded
the various sims games
Lord of the rings games
Ninja gaiden (there's another new ninja gaiden game out)
Soul calibur 2
DOA ulitmate
Destroy all humans
Samurai warriors (and other games of this type)
Kingdom under fire (and the recently released Heroes - for the strategy fans)
Don't forget to check out
http://www.gamespot.com/xbox/index.html for reviews/screenshots