Short tracks are something special. There's not enough time to slowly add layers and build to a climax. The message must be delivered instantly.
Unless it's some sort of filler in between longer tracks, it has to be direct and powerful... it has to be a real piece of music on its own, independant of other tracks... and memorable despite the short length.
And the winner is: Vangelis - Blade Runner (End Titles)
I haven't found any serious competition to my taste.
So, what's yours?