Not even "The Mists of Avalon" can touch it.
Arthur himself narrates, and while his voice and sensibilities may strike some as too modern and cynical, he fits the portrayal of a chaotic, Romanized society awaiting its inevitable doom. Arthur also provides an immediate hook into the story, which contains some of the most recognizably human characters I have yet found in an epic. Finally the knights are real, the whole bloody lot of abrasive, pigheaded men torn between loyalty to their clans and to the whole of Britain. Finally the women are real; Morgana is a very interesting twist on a Faerie queen, and Guinevere, long cast as a scheming adulteress or a weepy deadweight, at last stands as Arthur's equal and his most worthy opponent. As Arthur says, most kings have wives, but he had a queen. (Btw, if you like her here, read "Beloved Exile.")
The tale is a bit nonstandard, in that Arthur's father Uther is merely a Romanized noble, not the king of Britain; Arthur suceeds Ambrosius directly. Merlin is mostly absent, as is any overt magic, and when he does appear is anything but a bearded old man. Religion is largely a catch-as-catch-can issue in the complex, often self-destructive British society; there is also no Grail.
Instead, we get a look at a gritty, tumultuous period in the history of Britain through the eyes of a flawed, ambitious man who develops vision and compassion while stumbling towards true nobility. I cannot speak for historical accuracy, but the way things fall apart is stunning in its subtle inevitability; the characters react to each other and their environment in ways that seem natural, not forced by a preordained plot. Arthur and Guinevere's last effort to redeem themselves and patch things together has such desperate, moving potential that I find myself pleading with fate each time the story marches, naturally and relentlessly, to Camlan, where Modred fulfills his destiny.
And the ending is priceless.