Sword in the Stone (1963)
Based on the first book of T.H.White's tetralogy, which was the best version of King Arthur in the twentieth century, in my opinion. Everything good in this version comes from the source material.
Based on later parts of the same book! My favorite moments are when Lancelot brings back to life the knight he has accidentally killed, and when Arthur lays dying and says “Don't let it be forgot, that once there was a spot, for one brief shining moment, that was known as Camelot.” That brings tears to my eyes.
Monty Python and the Holy Grail (1975)
A hilarious if really uneven spoof. More quotable lines than any other movie. Probably too quoted. The truth is, in some ways this is closer to Mallory than most versions, in the way it seems to wander all over the place fighting and not going anywhere with the plot.
Fantasy movies in the 1980s were really hard to do right. The desire was there but without CG, it was really difficult to do believable magic. This is probably the truest to Mallory, and its devotion to mythology makes it seem strange. I like the super-heavy jousting plate they all wear all the time, though of course it's historically far too late.
First Knight (1995)
I adore the scenery, acting, and costumes in this version. I could watch Sean Connery, Julia Ormond, and Richard Gere all day. The plot is totally ruined and the characters are all wrong as they try to make it more Hollywood, though.
Merlin (1998 miniseries)
What a mess. The depiction of the villains was so badly done I still get the shivers thinking about it. Acting all in front of green screens, and changing the ending to make it happy. It had a few moments but was overall a failure.
King Arthur (2004)
This was an attempt to dramatize a plausible historical interpretation of Arthur as a post-Roman warlord struggling to keep civilization alive. It features Keira Knightly as a warrior Celt version of Guinevere. Good riding into battle scenes. Christians in the film are horrible people, just because Hollywood.
Merlin (BBC One)
I might have liked this better when I was younger, but I just don't like TV that much, so I only watched a few episodes. It's too modern in attitude. It seemed kind of like a Jocks vs Geeks high school series set in the middle ages. But I might not have given it enough credit.