Excalibur is perhaps the least historically accurate. And for all that, it's probably one of the most visually stunning.
Famed among aficionados for the scene where the disguised King Uther has sex with Igrayne while wearing full plate armor, this film sort of takes all the bits and pieces of the Arthur legend and plucks out whatever it damn well pleases to make a story. There are pieces of Thomas Mallory, T.H. White, Marion Zimmer Bradley, Geoffrey of Monmouth, Alan Jay Lerner, and a lot more besides to be found here.
The film starts with Uther Pendragon struggling to bring all of England under his rule. Merlin (who, tellingly, refers to himself as "the Merlin" at one point) has worked for years to bring peace to the land, and gives Uther the sword Excalibur to cow the reluctant warlords to accept his rule. Alas, his victory is short-lived, as lust for the lady Igrayne, wife of the duke of Cornwall, leads him to break the hard-won truce. Arthur is fathered, Cornwall is slain, and eventually Uther himself is assassinated, but not after he embeds Excalibur into a boulder, stuck there for decades.
More years pass, and after Lancelot and Guenevere betray Arthur by sleeping together (at which point Arthur loses Excalibur), and Morgana (Arthur's half-sister) not only orchestrates the downfall of Merlin but fathers a son (Mordred) with Arthur, the quest for the Holy Grail, which would heal the now-wounded land, begins. Morgana, it turns out, has suborned or slain the various knights who have stumbled upon her lair, but Percival overcomes them and eventually finds the grail.
One of the most outstanding features of this film is the performance of Nicol Williamson as Merlin. With his rolling baritone voice and skull-fitting chrome helmet, he embodied the character of Merlin for me for years after seeing him. The score draws heavily on Wagner (the Ring cycle, Parsifal, and Tristan and Isolde), and works very well for such a ponderous movie.
I say "ponderous" here not in an entirely negative connotation. The film has a very "heavy" feel, partly because of the costuming (the armor worn by the knights is a very unhistorical solid iron and later chrome-plated full plate armor), and also because of the almost oppressive feel of some of the locations. Woods are always deep and thick, the castles are cyclopean in their design, and even the parts in the wastelands are dark and claustrophobic. Liberal use of fog adds to the feeling. Wagner's music feels right at home in the setting.