Reasonable on 12 April 2011
I guess you've got lots of recommendations already, plus I note you have a lot of 360/PC titles on PC already particularly Mass Effect and Valve titles (good man, so do I) so I'll just say the 360 only titles I'd underscore are:
Gears 2
Lost Odyssey
Limbo
Fable 2 / 3
Halo 3 / Reach
Shadow Complex
Alan Wake
Viva Pinata
I tried Gears 1 on PC and it was okay but playing Gears 2 on 360 you can really tell they designed it for the console/controller.
Try to be reasonable... its easier than you think...
