Most of the Knights of the Round Table, Arthur included, should be either Cavaliers (Order of either the Dragon or the Lion) or Fighters. Those that are mentioned as being champions in the joust or with the lance would be Cavaliers, while others (like Kay) would be Fighters. Only a select few would be Paladins, as their holy talents were miraculous, not commonplace. Lancelot, Galahad, and Percival are the obvious choices for this.
Mordred is NOT an anti-paladin, but a Cavalier of the Cockatrice. He had no magic his aunt didn't give him as a boon, and was a war-leader and fairly good tactician.
Merlin should properly be a druid, except that wild-shaping doesn't show up in the stories much (Disney's Sword in the Stone notwithstanding). Nimue and Vivienne (the Ladies of the Lake) would be Sorcerors with the Fey bloodline, and Morgan is definitely a witch. Many stories say that she dallied with strange and unknowable forces from either the Faerielands or from Hell, and Witches cover that part of the story very nicely with their familiars.