You may pick just one feature film to be included in the Golden Record on the next interstellar space probe. It must represent what mankind is capable of doing in the motion picture form; which is why Forrest Gump cannot, or should not, hitch a ride on a vehicle that may go on its forever journey -- eventually to be found by another race of beings. (Now that I think about it, Forrest Gump himself might be a prime candidate for the trip to somewhere, some millennium, never to be seen again; at least not by humans.)
Back to the probe: My own pick might just be:
Metropolis, Fritz Lang's 1927 epic. The imagery is so forever, the film as a whole, so quoted and referred to, that, to me, there is no better representative feature-length motion picture.
A few years ago I got into a discussion with a friend about the matter and he said his pick would be 2001: A Space Odyssey, another "forever" piece of film art.
As much as I like Annie Hall, Bicycle Thieves, and Patton, I don't feel they best represent the 'bandwidth' possible in the art form.
What? Plan 9 from Outer Space? I had forgotten about that one....