Well, aside from this one, there are essentially three major Arthurian prose works. But to be fair, we'll extend it to other media. So in addition to Whites Once and Future King, Bradley's Mists of Avalon, and Stewart's Merlin Saga, we'll add the Musical Camelot and the film Excaliber. And while they're probably not in quite the same league from the perspective of Arthuriana, we should also throw in Lawhead's series and Monty Python and the Holy Grail. There are others, but the most significant of these (like TNT's production of Mists and Disney's Sword and the Stone are adaptations of those already on the list, so I'm going to exclude them partially for convenience and partially because they're too derivitive to get "greatest").
Of these, I would say the best done are Excaliber and White's book. The former for it's relative adherance to Mallory's version while still maintaining new artistic direction, and White for his drastic reinterpretation and wholesale renovation of the story. He made the Once and Future story once again, pulling in a whole new culture into it's application.
Now I'm not going to tear down most of the others. But Bradley's was, imho, really poorly poorly written - though it was frought with good ideas. Camelot I have a hard time sitting through. Maybe because I don't think it takes it as seriously as I like, though there's no denying it addresses things more seriously than would be immediately obvious from a musical. Stewart had some good ideas, but she makes some poor jumps when she gets to the end. Lawhead I haven't finished, so I'll reserve judgement. As for Monty Python: "Let's not go there. It's a silly place."
I'll just tack on at the end that I really enjoyed Hallmark's Merlin, but some of the production values interrupted me at times. Still worth watching. There are also some good children's works.
Now, again Lancelot. I hate to jump to his defense, because I, as well, don't like him. But I still can see how a good man can fall. For two reasons. One, sometimes he has just one weakness. He manages to reign in all his other passions but that one. The other is more complex: the late medieval concept of the knight is torn three ways. He's given three not always compatible loyalties: His lady, his lord, and THE Lord. Often a knight can pull this off. Lancelot, sadly, cannot. Devotion to his lady violates loyalty to his lord and to God. Devotion to either violates his loyalty to his lady. How can he pull it off? well, he doesn't, and we probably think he chooses the wrong loyalty, but to be fair, let's look at another story (one we'll see later in the series). In Tritram's story (Tristan), he falls also for the woman intended for his King (who also is his father), and they run off together. In the story, however, we clearly sympathize with Tristram, who is definitely more like Lancelot than any other character. True, Tristram and Iseult fall in love BEFORE the wedding, but well after the betrothal, and they both know. In addition, in most versions, the characters are the victims of a love potion. However, the analogy still works. Plus, how many of you would choose George Bush over the woman you love? For that manner, your best male friend over the woman you love? Ok, and even at that, some of you would, but the choice wouldn't be easy.
So yeah, Lancelot is realistic, I think. But we want our heroes to be better than us. To make good decisions without hesitation. But we find them so much more interesting when they're torn, and the interest perseveres for millenia at least in part BECAUSE of the Lancelot aspect. That's why people have recently found Superman to be dry, and why DC has started to try and make him more edgy.