1. Portal 2
Valve's huge success with Portal allowed them to continue the series with Portal 2. The game follows the character Chell right after the events of Portal who once again must pass the test chambers that she worked so hard to get through during the events of Portal.
Portal 2 plays exactly the same as Portal with a few more improvements and additions. You and your trusty portal gun must get from point A to point B. Sounds simple enough, but some of these levels are far from it and will require a bit of thinking. Using the portal gun will allow you to traverse the trapped rooms as well as pick up various items in the game.
With the addition of co-op play, you and a friend can take on even more test chambers as Atlas and Peabody. You can either work together, or make each other red with anger as you both sabotage the other's efforst to get through each chamber. The title is a spectacular puzzle game with superb level designs.