I won't say the most faithful or best, but the one I like best is T. H. White's The Once and Future KIng. Twain's A Connecticut Yankee in King Arthur's Court is a sharp-edged satire, but I read it so long ago I don't know if I'd still like it. As for movies, I like parts of Excalibur quite a bit and Bresson's Lancelot du Lac, altho it is depressing and a slog to get through.