Portal

Seriously, don't click on this video. The game is only about 3 hours long and you should really just do this on your own. Here is a list of things are less satisfying than playing through Portal and watching this ending on your own: graduating high school, getting through puberty, saving an endangered species, living a full and rewarding life, telling your parents you love them, getting your first leading role in a major motion picture and learning to play backgammon.