Second (or third, or whatever) Super Meat Boy, this is one of the only platformers I'd rate among Nintendo's best.
I'd also recommend Shadow Complex, as it's one of the best Super Metroid clones there is.
For retail, my favorites were The Orange Box, Red Dead Redemption, Assassin's Creed: Brotherhood, Bulletstorm, and Vanquish.
Honestly, there weren't a ton of games I thought were particularly exceptional the system. Like, Gears of War 2 reaches some campy heights and has a lot of variety, so I thought it was pretty good, but otherwise don't find the series interesting. Catherine is one of the more bizarre games I've played, but I don't think it was actually that good. Deadly Premonition is fascinating, but an absolute piece of **** to play.