It is hard to do such a feat unless you purposely stay clear of any hype and try not to keep up with the info.
Deus Ex: HR and Sleeping Dogs, I didn't really have too riding on those being good, infact Deus Ex wasn't even on my radar, decided to buy it and it was a f*cking amazing experience.
Minecraft is a big one as well. I barely knew what the thing was before it was out on the 360, I had no idea it was so popular, I grabbed it only because a few other people were banging on about it. So I thought, if it is co-op at least I might get some fun out of it. Turned out to be, well, it's minecraft.
I don't know if I can list Fallout NV, Blood Dragon, Skyrim, Rainbow Six Vegas, etc. Because I already knew they would meet my expectations.
However I guess they did exceed that as well so I guess you could list those.
Fallout 3 for sure, as I really was on the fence, I didn't buy it on launch, left it till the year after. Got it and was not fully convinced if I would like it.
Turned out to be probably the best game of all time, and is still the most gameplay you can get out of any one game I have ever seen.
Blood Dragon exceeded my expectations, but I already knew it would be amazing, so having it exceed that pretty much confirms it was one of the best games of all time.
Payday 2 as well. I didn't know much about it, I really wanted the first but I think there was mixed feelings with it. It also never came out on 360, the 2nd finally did. And boom. Game of the year for me with Blood Dragon.
So yeah, those.