La Jetée (Chris Marker, 1962)

Additional nominees for this collection: Metropolis (1927), Seconds (1966), Hard to Be a God (2013), Computer Chess (2013). (I'm not sure Hard to Be a God exactly qualifies as science fiction, because the setting is medieval and my recollection is that the actual premise is so buried that I'm not sure you'd even know what was happening unless you'd read a synopsis before watching it. But the borderline incomprehensibility of the plot aside, boy, it is an EXPERIENCE, and I've never seen anything else quite like it. Computer Chess is more billed as a comedy but is striking and memorable in a completely different way.)