The best science fiction movie of all time is a short (half-hour) black/white French film called "La Jetée" [translates as the airport's observation deck]. It is in the format of scanned photos with narration, like Ken Burns' PBS documentaries.
A child sees a man crumble and die while visiting Paris Orly airport's observation deck in 1962. Shortly after there is total nuclear war. Because he is obsessed with this image of a man's death, he is selected to be a guinea pig in an experiment to send him time traveling into the future in order to get an energy source to restart civilization. He succeeds in moving in time, but always ends up in the pre-war era. There, he meets a beautiful woman and falls in love.
It doesn't sound like much, but it is a true masterpiece. MIT even published a coffee-table book detailing every scene.
It is super low-budget. One scene that shows the Arc of Triumph in Paris with a huge chunk blown out of it actually has a pin hole from a thumbtack displayed in it.
David Bowie did a homage to it in video for a song from his Black Tie/White Noise album in the early 1990s.
It is available on DVD from most big-city library systems.