The British Film Institute (BFI), as part of its 75th anniversary has asked a number of film industry insiders to name the film that they would most like to see passed down to future generations. You can check out the Guardian article on the story here: http://www.guardian.co.uk/film/2008/sep/03/1. (Since it's a British project, it may come as little surprise that there are many British films cited.)
But I'm curious to know how you'd answer the question. It's a little different from "what's your favorite movie?" It's more like: if all the movies ever made were to be destroyed completely and irrevocably, what one movie would you save for posterity? And why?