I have:
Bayonetta
Devil May Cry 4
Fatal Inertia
Gi-Joe
Halo 3
Halo Wars
Halo: Reach
Lost Planet 2
Open Season
PGR3
PGR4
Project Sylpheed
Raiden Fighters Aces
Resident Evil 5
Street Fighter 4 (although I'm ashamed I own this garbage....)
Samurai Showdown Sen (even this is better than Street Fighter 4!)
Tony Hawks: Proving Ground
Other games I've played but do not own:
Bioshock 2
Assassin's Creed
Call of Duty: Modern Warfare 2
Call of Duty: Black Ops
Darksector
Dead Space 2
Gears of War
Gears of War 2
Halo 3: ODST
Mortal Kombat vs DC Univerce
Splinter Cell: Conviction
(I've also played some Fifa and Madden games though I forget which ones :P)
Planing to Get:
Brink
Crysis 2
Marvel vs Capsom 3
Raiden IV
Vanquish
I've only been gaming Xbox 360 since last year when Halo Reach came out, September 19 something like that... and I got to say, Xbox 360 is a pretty well crafted system!