10: Left 4 Dead 2...Left 4 Dead 1 just missed this list, but the second game is better in just about every way, with more variety in level design, weapons and enemies.
9: Dead Space...One of the surprises of this generation, a game that sucks you in like no other....except maybe Bioshock.
8: Gears of War 2...another game where the first game was amazing, but I liked this one just a bit more. To bad about the crap last boss though.
7: Assassins Creed 2....I did not want this game to end, and I love the progression of your main character.
6: Rainbow Six Vegas 2...A game where the multiplayer overshadows the solid single player. Terrorist hunts on veteran...a stroke of genius.
5: Saints Row 2...best sandbox game ever...
4: Bioshock...A game experience that sucks you in, and never lets go.
3: Borderlands...Biggest game surprise of all time.
2: Mass Effect...If not for the repetitive side missions, this could easily be number one, even with that slow [bleep] elevator.
1: Fallout 3....No words necessary.