Portal is a two-hour puzzle game that is also one of the funniest, most formally inventive, and narratively complete experiences the medium has produced. It started as a student project and shipped as a bonus in The Orange Box, and it still outclasses most things that ship as full-price releases.
The portal mechanic is introduced incrementally, given enough room to let you feel clever, and then combined in ways that reward spatial thinking without ever becoming obtuse. GLaDOS is the best written AI villain in games — comedic, sinister, and genuinely unsettling in ways the sequel would lean into further — and the environmental storytelling in the back half is some of Valve's best.
It's short enough that it never outstays its welcome, which is a discipline most games in the genre don't bother exercising. Portal does exactly what it came to do, in the exact amount of time it needs, and then ends on one of the best final moments in Valve's catalog.