Multiplayer: Payday 2. The original Payday: The Heist was basically Left 4 Dead + the first 10 minutes of The Dark Knight, & the sequel’s even better assuming that’s your cup of tea.
Single-player: I cannot keep my hands off X-Com: Enemy Unknown. Normally I’m not a strategy gamer, but the amount of customization for your anti-alien defenses are amazing! A friend said I like it so much because it let’s me pretend I’m Nick Fury, and all I can do is be mad @ how right he is ^_^;;