In honor of JFK's 100th birthday and the new film in theaters, here's an attempt to list all the films based on the stories of the Matter of Britain that are listed on Letterboxd. In addition to straightforward adaptations of King Arthur's life, there's also movies based on the connected legends and stories such as Perceval and the Grail Quest, Tristan & Isolde, and A Connecticut Yankee in King Arthur's Court.
TV versions are only included if they're on Letterboxd, natch. I was able to further fill out this list with reference to the Camelot Projects's comprehensive listings (read them here under "Arthurian Film" and "Arthurian Animation").
Suggestions for ones I've missed are welcome, as well as information on what the heck Merlin and Arthur the Lion King is.