Phantasy Star Online(on the awesome dreamcast)...
All Resident Evils, particularly 2...
Tomb Raiders 1,2,3 & Underworld...
Zelda Ocarina, Link to the past & Wind Waker...
Guild Wars...
Mass Effect will probably top the list once the whole trilogy is out, if my time on the first game is any indication...
Playing: Borderlands(great co-op,HUGE amount of content),Too Human(better late than never lol),Saints Row 3(Penetrator ftw),Minecraft 360,Harry Potter Lego.
Patiently waiting for: Tomb Raider, Borderlands 2