This is honestly one of the hardest list to come up with... so ill just throw a somewhat coherent list together.
Super Meat Boy - a very addicting game because of the ease of one last time retry.
Hotline Miami - for the same reason of SMB, I spent countless hours attempting to master a level.
Mass Effect 2 - I attempted to play 1 & 3 but 2 was so hooking on in-game "choice" and I loved the combat/gameplay the game provided.
The Stanley Parable - I love the narration and non-choice the game provides. I have spent a good amount of time thinking of attempting different endings. This game also put a smile on my face everytime i play it.
Uncharted 2 - The graphics literally felt the next gen came. The graphics were supported by beautiful set pieces and great gameplay. I remember the opening sequence and i had my controller down, then i wondered why drake was not moving.
Viva Pinata - In the lime light of Gears of War 1, I remember playing this game more and having a lot more fun. The art/graphics were also beautiful.
Battlefield 3 - the gameplay wasn't that great compared to other battlefields but this game hits personally level by playing with a friend for 200+ hours MP on PS3.
JC2:MP - I recently started playing this but I cant keep a smile off my face with playing in a super boost server.. especially in a derby match.
Skate Series - I cant single one of the games out because each had its highs/lows. From Skate 1 introducing the flick-trick stick. 2 creating a great world to skate . and 3 with hardcore mode and park creation, but I know I sent countless hours playing each game of the series.
Team Fortess 2 - I started playing this game before the hats and micro transactions.. And recently (last month) got back into the game and I still love the gameplay even without buying items/crafting. The gameplay and feel is solid and hope it will stand the test of time.
Left 4 Dead Series - So many great personal memories of playing with friends and built with solid gameplay.
Call of Duty 2 - I remember seeing this game played at a B&M kiosk, and saying to myself "This is truly next gen"
Call of Duty 3 - more of a personal memory of being on top of the leaderboards for medic and capture the flag games. I could not get enough of the multiplayer and alot of great people on X360
Portal Series - The cleverness of writing/narration was top-notch at the time and i loved the gameplay.
Red Dead Redemption - What a beautiful game but I do hate Mexico..
Catherine - Solid Puzzle Gameplay with a weird but gripping story.
Rayman Origins - Beautiful Art Direction with stellar gameplay
I know I am probably missing a ton of games that I love this generation, but this generation felt it lasted a life time and I certainly cannot catch every game..