Interesting item at io9. They set up some brackets with the top 64 science fiction movies of all time. They've already done a couple of rounds so the choices are whittled down but it's interesting to see what current fandom thinks. Also interesting was the seeding that the io9 staff decided on. Their top four seeds:
2001: A Space Odyssey
You can go there and vote if you're interested. No registration or anything needed.