Dead Space 2. I liked the original, but wasn't blown away by it. DS2 was done the way a sequel should be done: they actually fixed what was dodgy in the first game, added more content, but kept the gameplay (almost) exactly the same. I think it's a stretch to call it *great*, but it was very good, a pleasant surprise.
I was surprised by Darksiders also, but when the developers went nuts with the "ray-beam puzzle orgy" at the end I shelved it, didn't finish it. I play hack'n'slashers to do exactly that, hack'n'slash. Not spend my time screaming at the monitor over a tedious puzzle some developer put in there to make themselves feel clever.
A similar game that was a nice surprise, bordering on great, was X-men Origins: Wolverine.
Magicka. I thought it'd be little more than a day's diversion, but it turned out pretty good.
The one game that completely blindsided me with how good - and how addictive it is - is Plants Vs. Zombies. I *still* play that game, and probably will for years.