I finish every game I buy, just because I want my damn money's worth. They may not be easy, but at least I'd never cheat at one. OK, that's a lie, I use cheat codes, but I always beat the game once before using them. It used to be because I didn't want my saved files to get corrupted, but know I just do it to validate my gamer-n3ss. But I'm getting off topic, mine would probably be Portal. It was a tad short if anything, but pretty funny and definitely inventive. You need to be a little creative to figure out a couple of the puzzles, but it doesn't really require any skill, and very little timing.
EDIT: After mastering GeoW on Hardcore, Casual is nothing for me. Not even the Wretch levels are even scary. I basically sense that they're there, and know when they're gonna come. Gears'd be a close second.
DOUBLE EDIT:
CrazySlyHawk said:
Probably AssCreed
Very easy, in a good way of course. Very fun!
Most of the game is pretty straightforward, but if you've ever faced sixteen guards at once, you can tell it's not that easy.