If I had to choose, I'd say TF2. But a lot of that has to do with the awesome people I have to play the game with, so after that would be a tie between:
Deus Ex: There isn't a lot I can say that hasn't been said a bajillion times. Play this game, and they had BETTER not screw up Human Revolution
Demon's Souls: I loooooooooove this game. I love the atmosphere, I love the way every weapon feels different, I love the controls, I love that it's challenging yet fair, I LOVE how the multiplayer forces me to stay on my toes, I love the bosses, I love how much of a badass it lets you feel like. There's a few minor gripes I have with it, but the positive FAR outshines the negative.
Deus Ex: There isn't a lot I can say that hasn't been said a bajillion times. Play this game, and they had BETTER not screw up Human Revolution
Demon's Souls: I loooooooooove this game. I love the atmosphere, I love the way every weapon feels different, I love the controls, I love that it's challenging yet fair, I LOVE how the multiplayer forces me to stay on my toes, I love the bosses, I love how much of a badass it lets you feel like. There's a few minor gripes I have with it, but the positive FAR outshines the negative.