Portal 2

One of the most brilliant and original games ever was almost an afterthought. Developers weren't sure what to do with Portal, a mind-bending puzzle game that lasted only a few hours, and eventually lumped it in as bonus content with the best-selling Half-Life series, where it became a cult classic. In Portal 2 (360, PC, PS3), the sequel and first full-length game, main character Chell returns to Aperture Laboratories to face off against GLaDOS, a hilariously twisted artificial-intelligence system that delights in challenging her to escape chambers loaded with lasers, fans and other hazards. Chell's only weapons are a gun capable of creating a portal between any two flat surfaces and surface-altering paint that allows her to bounce off watts and run faster. The gameplay is genius, forcing you to think dimensionally to find a way out of each puzzling chamber. The new co-op mode allows players to work together as a pair of robotic characters as GLaDOS attempts to turn them against one another. It's part WALL-E, part brainteaser and possibly the game of the year.