I voted for Mass Effect.
I didn't really like how Valve neglected the difference between gamepad and mouse. Doesn't hinder gameplay, but it's quite annoying when aiming for achievements.
(Go on youtube, search Portal challenge mode videos on both versions and see how PC version is "click here, click there, voila!" and 360 version is "may the Force be with you, Jedi")
Another strong point of ME is it's 360 exclusive (at least for now, the "Only on 360" logo is total bullcrap, those games eventually get ported to PC, PS3, everywhere) so its graphics and controls aren't challenged by AC,OB,COD4 running on high end PC.