Portal 2
Never mind the ingenious permutations of bouncing momentum, gravity and light required to progress through this game. Portal 2's biggest surprise came from experiencing how developer Valve left the cool, minimalist remove of its predecessor behind for a messier, more emotionally textured path. Where the first Portal delivered an antagonist for the ages in bitchy AI GLaDOS, the follow-up peels back the layers of her computational psyche and shows us how she got that way.
As you plumb the conjoined secret history of GLaDOS and mad-science corporation Aperture Science, you feel not only your mind expanding but your heart too. Long after the physics-manipulation conundrums have been solved, the poignancy of Portal 2's man-and-machine pas de deux lingers on.