I really can't list just one. My body won't let me.
Mass Effect was probably the first game to blow my mind last gen. The story was so epic and the universe they created was so brilliant. I felt as though the thought to make ME2's story more personal and contained kinda killed off some of the hope I had for the series. Then ME3 shyt on the rest of the hope I had left over. Note, not saying either are bad games. In fact, ME2 I beat 4 times. Just saying that the world that ME built and the final 2-3 hours of that game made me feel as though gaming pretty much found it's Star Wars. Still love the series but was let down.
Call of Duty 4 will probably never be topped IMO when it comes to pure fun factor in a shooter. Even with the P90 spray fests, the grenade chucking, and fukking juggernaut, the game was ridiculously balanced. The last of these CoD games to have a story worth a damn as well before the series turned into fast food.
And Far Cry 3. Game snuck up on me out of nowhere. FC2 was ass cheeks so I didn't really expect dikk from it. But...easily my favorite game from the past 2-3 yrs.