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....