I'm late so I doubt this will help you now but fuck it, I like lists
Retail-
1- Halo 3
2- Halo Reach
3- Alan Wake
4- Gears of War 1/2
5- Halo Wars
6- Fable 2/3
7- Crackdown 1 (don't bother with 2)
8- Mass Effect 1
9- Splinter Cell: Conviction
10- Lost Odyssey
XBLA. I have a criminally low amount of XBLA games but these are good
1- Shadow Complex (getitgetitgetit)
2- Toy Soldiers
3- Dishwasher: Vampire Smile
4- Geometry Wars 1/2
5- Duke Nukem 3D
6- Worms 1/2
The rest- Sonic 1-3 & Knuckles