One of the most important puzzle games to ever release. The characters absolutely make this game. The constant banter between GLaDOS and Wheatley is a sight to behold, and the puzzles are paced fantastically.

The game also offers a co-op mode where you can solve puzzles with a friend, and the game supports Steam Workshop for modding support.

This is one of those games that almost anyone will tell you is a "must play" when you first create your Steam account for the first time.