Not totally scientific, but I took the top 25 List Challenge lists of a general Science Fiction films-based nature; i.e., basically anything identified as "Sci-Fi" (but not a specific period or "type" of Sci-Fi, like did not include lists looking at just Space movies), all with a User base of at least 2,100, and tabulated a total sum of each movie's % of users value taken from each list's Stats tab. The films were then ranked in order, highest to lowest, based on their total % sum. I will publish a 2nd list soon ranked by total number of users, calculated by taking each list's user % times the list's user base.
Whether a film is "Sci Fi" can be a subjective choice; even "A Clockwork Orange" is at #25 (more of a dystopian genre), showing up on 16 of 25 lists, so the appearance of films on THIS list is driven by the inclusion on the 25 lists used to create the top Sci-Fi watched. I'm sure there will be future Watched Lists specifically looking at Space or Fantasy or Dystopian genres.