#14 Braid
Braid is Jonathan Blow's masterpiece and gift to the gaming world. Developed on the ever malleable concept of time, Braid is split into multiple different 'worlds' in which a certain ruleset applies throughout the levels.
The basic premise allows Braid's protagonist to stop time and turn it backwards, preventing deaths and otherwise allowing him to surpass insurmountable obstacles. Further levels take the concept to a whole other level, like a world where time only ever runs backwards, and the player's power allows you to move forwards.