I'm an admitted King Arthur nut. I've read Mary Stewart's books, T.H. White's classic The Once and Future King (including its oft-forgot final chapter, The Book of Merlin), and yes, even slogged my way through some of Malory's Morte D'Arthur. I own Boorman's Excalibur, which is a mostly excellent film, and even "borrowed permanently" Dad's copy of the 2004 film King Arthur.
I still have to read Marion Zimmer Bradley's stories, but Mists of Avalon is near the top of the To Read List.
Having hopefully established my bona fides, I can say without reservation that the best Arthur stories I have ever read or seen remain Bernard Cornwell's Warlord Chronicles, three books consisting of: The Winter King, Enemy of God, and Excalibur.
Cornwell is a consistently fantastic writer. His stories are vividly descriptive without being flowery; poignant without ever dipping into sentimentality; and sheer, visceral, rollicking good fun without ever turning campy.
I have read many of his other books, which are all good, but the Arthur books in my humble opinion are a cut above the rest. In these tales, Arthur is the king in all but name; Mordred is not his son but rather his half-brother; Lancelot is a gutless, manipulative, vain, back-stabbing, power-hungry cad; and the narrator and unabashed hero of the saga is a Saxon-born man named Derfel, raised by Merlin.
All three stories are thrilling, gripping adventures. If you've never read Cornwell before, his bread and butter is his ability to draw realistic battle scenes. Cliche as this is going to sound, you feel like you're standing in the shield wall with Derfel as the barbaric Saxon hordes, outnumbering Arthur's men by the thousands, charge. Life, even outside of the battlefields, can be nasty, brutish, and short, to steal a phrase. His characters are real people, heroic, flawed, often well-intentioned, and fighting to carve out a happy existence.
I can't recommend these three books enough. I consider them one of my two favorite trilogies, the other being Adrian McKinty's kick-ass Dead Trilogy, which I've posted about before.
11 hours ago