Guitar Hero: World Tour. Relax, I'm kidding.
Brutal Legend. Alright, alright, I'll be serious.
Quite possibly Red Dead Redemption or Mass Effect 2. And though I didn't get around to finishing it, I really enjoyed what I played of Batman: Arkham Asylum. I have no idea why I never got around to finishing that, and I still own it.
I did play World of Warcraft for a few years here and there and enjoyed it, but I can't figure why.
But for now, best I've played has to be Red Dead Redemption, no question. There's just so much to do in that game, it's ridiculous. Ridiculous in a fantastic way, of course.