io9 is doing a series on Science Fiction for Beginners. It includes a post today by Charlie Jane Anders, 25 classic science fiction movies that everybody must watch.. It's a good list.
I am embarrassed to admit that there are a couple of movies on the list I haven't seen. (No, I haven't seen Metropolis yet, and I realize that makes me culturally illiterate. Nor have I seen Planet of the Apes, Road Warrior, or Ghost in the Shell.) I'll have to make a point to see them.
I do agree with Anders about Brazil— when forced to list a favorite movie of all time, usually that's the one that I list. I'm also happy that both a Star Trek and a Star Wars movie made the list, because those movies (the second in each series) were good movies; sometimes people are too self-consciously highbrow to include something from a mass-market franchise.
I do have to quibble with what Anders says about Back to the Future. A very fun movie, mind you, but I wouldn't say that it's theory of time travel really makes all that much sense. I suppose it does apply the theory consistently, but it was definitely a "fantasy" theory of time travel. A science fiction movie that I think is great and that I'd include (along with Primer) as one of the two "essential" time travel movies is 12 Monkeys. (Which, I believe, was produced by Terry Gillam, who also did Brazil.)
Other movies that I would have considered for the top of the list include Gattaca (probably the most sensible treatment of the social spectre of genetically engineering our kids), The Truman Show, and maybe, just maybe, Buckaroo Bonzai, as the definitive and most rewatchable treatment ever of camp.