Portal 2 is a really good puzzle game, especially with its 2 Player Co-Op. The single-player is good too, but (for me, at least) the puzzles are pretty difficult when you're doing them all by yourself without a walkthrough.

Portal 2 is a really good puzzle game, especially with its 2 Player Co-Op. The single-player is good too, but (for me, at least) the puzzles are pretty difficult when you're doing them all by yourself without a walkthrough.