Even though it's mentioned that it's not this type of thread at the top, I just have to say I was very impressed with ME2 this year. I really enjoyed the first one, but the second one for me was one of my top games of all time. Even though it's not my most played game of 2010, I can safely say it got the longest gaming session out of me this year. One session I spent 14 hours on the game, including short bathroom breaks and meals.
For most time spent playing a game, it's gotta be MW2, since I was trying to go for tenth prestige at a few points this year. I only made it to prestige level 5 or 6 I think.
Biggest surprise for me is a tie between Alan Wake and AC: Brotherhood. I put off playing Alan Wake til just a month or two ago for whatever reason. I heard a lot of good things about the game, but even that couldn't prepare me for the awesomeness that is Alan Wake. As for Brotherhood, I thought I would get bored of it quickly since it's so similar to AC II, but instead it was one of those games I kept putting in everyday because it was so much fun. Oh, also NBA 2K11. I know the 2K series have always had great basketball games, but 2K11 kicked that up a notch and is one of the best sports games I've ever played.
My biggest disappointment has to be the Rock Band 3 pro keyboard. It started out pretty neat, and I even learned how to play some songs off of it, but ultimately I became bored with it and I haven't played it since Halloween. I think a lot of that has to do with the fact that I play actual keyboard, and only playing one hand's part of a lot of songs has turned me off of it.