Call of Cthulhu, because it gives full leverage towards character death, appropriate to the genre.
Paranoia, because its a perfectly set up scenario structure that can be replayed indefinitely, that involves no character development whatsoever to generate a good time (and up to six character deaths per player!).
Ghostbusters, for being the first, and arguably still the best rules light system with an easy-to-engage-with light hearted scenario set up.
Unknown Armies, which actually has an excellent series of One Shot scenarios in this slim little classic.
Puppetland, which lasts precisely one hour and is a curiously perfect little diceless game.
Toon, for a night of inconsequential absurdity.
HoL: Human Occupied Landfill, for a night of inconsequential, offensive absurdity.
Feng Shui, for being an easy to introduce pick-up-and-play, kitchen-sink action movie game.
Prince Valiant, for having a system so simple, even children can literally pick it up and play.
The Whispering Vault, which is entirely designed and set up for one shot scenarios.
Doctor Who, which is also entirely episodic in scenario set up, with a TV show for inspiration.
Traveller, for having random tables set up for lazy scenario designers.
Fiasco, for having random tables set up for designing scenarios in any genre.
Your Favourite System, because lets face it, any game that you are familiar with can be used for a one shot as you wish.