Heres some 90%+ games you should definately consider.
Mass Effect - Crossover RPG/FPS gaming fun.
Bioshock - Awesome gameplay FPS.
Halo 3 - Awesome Local 4 player multiplayer/Online, easy to drag your friends into playing because its easy to play.
CODIV - Same as above, for online I would go CODIV, for Offline I would go Halo 3.
Some 80+ Games you should buy if you like their genres.
Ninja Gaiden 2 - The best hack n slash out there right now for this gen. Hard but very cool. Has Coop afaik...
Condemned 1/2 - Bargain bin, cool games not sure about multiplayer. The combination fighting/FPS is pretty cool.
Lost Odyssey - Considered the Best JRPG out for the Xbox360.
Crackdown - cool arcadey GTAIV type affair, pretty fun but not played much.
Otherwise check that Games to buy for Xbox360 owners sticky in these forums for some awesome but cheap games to buy/play.
Tease.
