This is an easy one for me: ICONS. I've written a ton about it, I've played quite a bit of it, and it is still hands-down my champion for quick and easy play.
- If I couldn't play ICONS and had a short window I'd probably go with one of its parent games - Marvel Super Heroes, the 80's TSR RPG. Hand out some character cards and get to fighting evil!
- Another surprisingly quick option is another Marvel game: Marvel Heroic Roleplaying. Again, hand out sheets for Iron Man or Spider Man and go!
- Outside of this I'll go with "Savage Worlds" - characters are fairly quick to create and gameplay moves right along, especially with a session or two of experience and it feels like you're getting a lot done.