I've watched three King Arthur movies recently and it's been interesting to see what each one emphasizes.
First Knight features an old Arthur and focuses on the Guinevere-Lancelot love story. The Round Table is based on the principle of serving others and protecting the weak. First Knight has a simple fantasy setting where every bad guy is dressed like a highway bandit and every good guy wears velvet and silver armor. Everyone in Arthur's court wears blue. First Knight almost portrays a complex moral question when Arthur catches Lancelot with Guinevere, but the cartoonishly evil bad guy attacks before anything interesting is done.
King Arthur attempts to portray the historical Arthur, a half-Roman Briton who leads the Britons against the Saxon invaders once Roman forces leave Britain. The knights of the Round Table are Sarmation mercenaries. The setting is good and the knights have great chemistry among themselves. There's no evidence in this movie, however, that any of these knights actually like Arthur. Both Arthur and Guinevere have a habit of just staring at people as a substitute for conversation, so the romance between them is little more than a series of meaningful looks. This Arthur is mostly concerned with promoting free will and the right every person has to do what they want.
Excalibur follows a simplified version of the legend told in Morte Darthur, starting with Arthur's conception and going all the way to his end. Arthur starts as a young and energetic king guided by Merlin and then grows to become wiser and more passive. Lancelot is a perfect knight who fails through one temptation, and the Round Table eventually falls because of Arthur's and Lancelot's mistakes. Knights in Excalibur wear full suits of shining armor almost all the time.
It just so happens that I've arranged these movies in order of personal preference, with Excalibur being the most preferred. None of the three stand out as classics to me, but it's safe to assume that more Arthur movies will come out in the future. I'm personally waiting for a movie that focuses on Gawain, a character arguably much more interesting than Lancelot.