Quatermass said:
Greatest science fiction film ever? Well, that's very subjective. If we're talking influential, none can have more influence than Metropolis. Without that, no Star Wars, no Blade Runner.
Gotta disagree with you there. By that reasoning, you could argue that Metropolis wouldn't have been possible with Meleis' (sp.) "Voyage to the Moon." As great as Metropolis is, it never started a stampede of similar dystopian SF movies immediately after it came out, and Fritz Lang never did anything like it ever again.
Star Wars was far more influenced by the cheesy Buck Rogers and Flash Gordon serials of the 1930s and 40s and by Star Trek and Lost in Space, which were, in turn based more on the cheesy pulp SF space opera stories and novels of the 30s, 40s and 50s. Blade Runner was far more influenced by Stanley Kubrick's genius of mis-en-scene in 2001 and A Clockwork Orange than by Metropolis.
For me, 2001 is the best and most influential SF movie ever. Before it, most SF movies were either glorified space operas (Buck Rogers, Flash Gordon, Forbidden Planet), invasion/monster movies (War of the Worlds, Godzilla, Them, and its ilk), or commie paranoia (Invasion of the Body Snatchers).
2001 was the first movie that made the science part of SF the central theme of the movie, relegating 'humanity' in a secondary role. In terms of realism, atmosphere, and intelligence, combined with slow pacing that truly mirrored what the monotony of space travel would be like, it changed the genre completely.