360 exclusives, though they've already been covered:
ME is good, mostly. I just hope you can stomach driving the Mako.
Gears 1 and 2 are excellent. Even better coop... or if you dig the multi, there's a lot of maps.
Dead Rising is phenomenal.
You have the obligatory MS franchises of Halo 3/ODST and Forza 2/3. Haven't played much there except H3, which wasn't really for me.
Crackdown is excellent. Shame on the GGT for not pointing this out yet.
GTA IV: Episodes from Liberty City, if you liked GTA IV.
There's probably older games that may float your boat that just weren't available on PS3 at the time as multiplatform or those that have better ports. People say The Orange Box is better on 360. Also RE5 and Ghostbusters.
XBLA games, there's a lot there -- Geometry Wars 1/2, Shadow Complex, Dishwasher, Pacman CE, Galaga Legions, etc. I'm sure there's more, but those are the ones that immediately come to mind.