Very short and more of a tech demo or proof of concept for its later sequel, but obviously massively impactful on gaming as a whole and on gaming culture. Initial dawning realisation that GLaDOS is sentient and malevolent, and when you first escape the boundaries of the testing chamber, were amazing.

I'll never understand why lines like "The cake is a lie" became the over-memed repeated jokes when there are infinitely funnier and cleverer pieces of dialogue in this game, but so it goes.