I've read a lot of Arthurian retellings and have to count Bernard Cornwell's "Warlord" trilogy as one of the absolute best. It truly has it all.
The story adheres far more to the "historical" Arthur of Gildus, Nennius, Geoffrey of Monmouth and William of Malmesbury than the "romantic" Arthur of Mallory and Chretien des Troyes. The Welsh and Irish influences are also strongly felt. In Mr. Cornwell's "historical" treatment, Arthur is not a king, but a Warlord. Uther was Arthur's father, but Arthur did not share in Uther's legacy. Merlin is a well-developed character, but is a wealthy lord and full Druid. A cauldron, another powerful Celtic symbol, replaces the Holy Grail. Finally, the sword was on the stone, not in the stone.
But, the fact that the story can be told a multitude of ways, from a multitude of viewpoints, with a cast of both new and common characters points out the strength of the Arthurian legend and the Arthurian public's hunger for more. Each retelling worth its salt (and this one is!!) provides it's own unique take on one of the best-loved legends of the English-speaking world.
Even though Mr. Cornwell paints Arthur as a strong and (mostly) virtuous character, he still has flaws. His willingness to forgive his enemies sometimes defies logic. His modestly sometimes rings hollow. But, all in all, he is the heroic figure we all have come to know and love. The same cannot be said of Mr. Cornwell's Lancelot. Suffice it to say that this is by far the least flattering portrait of the "world's greatest knight" that I have ever read. Some of the other ancillary characters get a fuller treatment here than in many other versions of the tale and with some interesting transitions. Guinivere, at least in my book, undergoes several personality transplants during the course of the trilogy, and some are for the better. Nimue/Vivien is far more developed in this series and her transition also runs the full gamut. Many of the other nobles and warriors are also well-painted characters and add a great deal to the overall enjoyment of the story.
But, the true measure of an Arthurian novel lies in the strength and credibility of the narrator. Derfel is definitely one of the strongest voices I have heard in Arthurian literature. He is now an aged monk, but in his younger years was a friend and confidant of Arthur who often fought by Arthur's side. He is a sworn enemy of Lancelot, an angle that adds many new slants to the story. But, most importantly, Derfel is telling the story in the past tense; he alludes to future events, tells us enough to let us know where it all leads, and then fills in the blanks. In some respects, Derfel fills the role of Bedivere (which is all I'll say now so as not to spoil the story for those who haven't read the books yet).
The writing is powerful, the story is spellbinding, and it achieves that delicate balance of retelling the familiar yet introducing a healthy dose of the new. You just can't ask for anything more.