Internet Movie Database Users continuously Vote For The All-Time Top Fifty Science Fiction Movies
Interestingly enough, they calculate the top 50 (and bottom 10) using a formula based on ratings from their site, rather than just a straight vote from a list of movies.
Weighted rating (WR) = (v ÷ (v+m)) × R + (m ÷ (v+m)) × C
R = average for the movie (mean) = (Rating)
v = number of votes for the movie = (votes)
m = minimum votes required to be listed in the Top 50 (currently 1000)
C = the mean vote across the whole report (currently 6.6)
For the Top 50, only votes from regular voters are considered. In order to appear in the Bottom 10 Titles chart the title must have received at least 50 votes.
The only problem I can see is that some of the films are suspiciously un-scifi: Wallace & Grommit In The Curse of the Were-Rabbit? (That’s probably because they just happen to be put into the genre of ’scifi’ and lots of people voted highly.)
Note that the page is dynamic; as more people vote and movies are added, the list might, and probably will, change.
The bottom 10 scifi movies list is worth looking at too.