Good question. I haven't played a lot of them, my gamer life is low lately, but Mario Galaxy, as childish as I thought it would be, is awesome. The fact that you can get a ton of oh-my-god-awesome old-skool games makes it such a worthy purchase.
Call of Duty Black Ops - stay-up-till-4-am kind of awesome
Mario Galaxy 2: More fun that you expect
The New Super Mario Bros: I didn't think I could like a Mario game any more than the old ones....but I'm almost ashamed to say that this one might be second fave.