I find the statement about the two movies mentioned in this thread being among the worse movies ever "interesting" since both movies had multiple Oscar nominations and in the case of National Velvet, I believe it is considered a classic. I'm also not sure how "Ranker.com" became a "reliable source" for movie ratings and reviews?
National Velvet was nominated in 5 Oscar categories: Best Actress in a Supporting Role (Anne Revere), Best Film Editing (Robert Kern), Best Art Direction-Interior Decoration, Color (Cedric Gibbons, Urie McCleary, Edwin B. Willis, and Mildred Griffiths), Best Cinematography, Color (Leonard Smith), Best Director (Clarence Brown). Anne Revere and Robert Kern both received the Oscar for their work in that film. National Velvet
National Velvet also was selected to be included in the Library of Congress' National Film Registry in 2003 National Velvet The National Film Registry
The New York Times Review in 1944 said of this movie: "National Velvet," should be a joy to all right-minded folks. For this fresh and delightful Metro picture, based on Enid Bagnold's novel of some years back, tells by far the most touching story of youngsters and of animals since Lassie was coming home.National Velvet
BUtterfield 8 received two Oscar nominations: Best Actress in a Leading Role (Elizabeth Taylor), Best Cinematography, Color (Joseph Ruttenberg and Charles Harten) Ms. Taylor won the Oscar for that performance as well as the 1961 Golden Globe for Best Motion Picture Actress - Drama. She was also awarded 2nd place that year by the Laural Awards (1st went to Shirley MacLaine for "The Apartment") as well as the movie BUtterfield 8 and Co-star Dina Merrill were also nominated for Laurel Awards that year.BUtterfield 8Laurel Awards
The New York Times didn't love it but certainly didn't call it one of the "worse films ever". Butterfield 8
As far as I can tell Elizabeth Taylor and her then Husband, Eddie Fisher, are the only people of note at the time that thought the movie stunk.
I'm sure Elizabeth Taylor had to have appeared in at least one "stinker" in her career (I think most would agree that The Flintstones movie can't be considered a great movie on any scale) and Cleopatra almost bankrupted 20th Century Fox Studios, but I don't think the two movies originally mentioned can be considered in those categories.