My top 20, in no particular order:
1.) ST TMP: I just like the models of the Enterprise and the Klingon Battle Cruisers, otherwise, the movie is a complete waste.
2.) Star Wars, A New Hope (original edit) We'll never see that one or anything like it again. Without execption, it was the best SF film ever made. Unfortunately, the version available today is not even watchable. Star Wars, the Empire Strikes Back is the only remaining film in the series that could replace it on this list.
3.) 2001, A Space Odyssesy, the special effects in this film have yet to be duplicated.
4.) Terminator.
5.) Aliens, The second movie, not Alien.
6.) Forbidden Planet.
7.) The Chronicles of Riddick.
8.) Battle Beyond the Stars, I just like the model of the Corsair.
9.) The Planet of the Apes.
10.) Abbott and Costello Go to Mars, Hey, if I'll use any excuse to list Abbot and Costello.
11.) The Day the Earth Stood Still, Why they made such a horrible remake of this classic, I will never know.
12.) Godzilla, King of the Monsters! You have to see at least one Godzilla film.
13.) The Andromeda Strain.
14.) Silent Running.
15.) Westworld. This was a movie about a computer virus. The concept was completely alien to people, back them.
16.) Logan's Run.
17.) Spaceballs. The funnyist film ever made.
18.) The Fifth Element.
19.) Avatar.
20.) Robocop.