Portal

Valve's 2007 puzzle game began as a student project called Narbacular Drop and shipped as a two-hour bonus in The Orange Box, where it promptly overshadowed most of the box. The idea is one mechanic taken seriously: a gun that places two connected holes, so you fall in one and come out the other with your momentum preserved. Nineteen test chambers teach you what that implies - that a long fall becomes horizontal speed, that a cube can be sent somewhere you cannot reach, that walls are only obstacles if they are not painted the right shade. Nothing is padded. Sitting on top is GLaDOS, an artificial intelligence running the tests while lying to you about cake, whose steady slide from passive-aggressive to openly murderous is one of the great comic performances in games. It ends with a song that charted. Its influence is out of all proportion to its length, and it is still the standard example used when explaining how to teach a mechanic without a tutorial.