One of the best Science Fiction movies I think I've ever scene has to be Forbidden Planet.

It was the inpiration for practically everything that came afterwards in Sci-Fi film.
From Star Trek of the 1960s to Dune to Star Wars, this film covers nearly all of the major themes used in contemporary Sci-Fi.

Trailer:

Movie Poster:

SPOILER: promotional poster (pretty large image) 


If you haven't seen it yet, its worth renting.