Too many amazing games to choose from.
Although I have to nominate BioShock 1, it was the perfect game for me, it had an amazing storyline, well thought out characters, it had an amazing atmosphere creating a sense of depression and claustrophobia, it had an amazing plot twist, amazing final boss battle, amazing weapons, amazing plasmids, amazing enemies, and an amazing protagonist. Actually now I think about it BioShock is the only game I could nominate... and now I'm going to go play it because it's been a year since I last played it!
(other games I would have nominated, Batman, Prototype, Jak 3, Sly Cooper (the series), Ratchet and Clank (the first one))
EDIT: I would have also nominated Portal, World of Warcraft, Call of Juarez: Bound in Blood.