538 is using the wrong metrics. For a movie to be truly, truly, sublimely bad, that badness needs to be recognized - people in some cases will want to watch it for its incredible badness. This means that there are going to be a disproportionate number of both 1 and 10 votes. And the winner based on this is - The Room. http://www.imdb.com/title/tt0368226/ratings?ref_=tt_ov_rt
Unexpectedly, however, "Plan 9 from Outer Space" is rated much more evenly for a film that's been considered to be the worst of all time.