"The science-fiction movie genre is as expansive as the galaxy. Ever since the age when technology meant little more than a lensed metal tube that could peer into the sky, we've used technological advances to realize our dreams and nightmares. Now a model spaceship or two can turn a Western into a sci-fi classic; an experimental process is all a romantic drama needs to become far-thinking speculative fiction.
That's why ranking the best sci-fi movies of all time is a difficult, even dangerous task. There's a good chance your favorite movie isn't on this list, or isn't ranked high enough for your taste. Many massively successful spectacles, such as Avatar, didn't make the cut. Other films, like Edge of Tomorrow, blend concept and character with excellent, even ground-breaking, effects, but didn't quite make the top 50."