As I may have made clear recently, Star Wars is my favourite sci-fi film, if I had to pick one, It would be Empire Strikes Back, I think mainly because that film is the one which is at the top storyline wise (i.e the "I am your father" scene) I also like Revenge of the Sith, maybe because it makes the first 6 stories complete, I also liked Anakins turn to Darth Vader. I also like Aliens, I feel that was the best out of the whole "Alien" franchise.
What's your favourite Sci-Fi film?