Why is Lancelot stated to be the best knight of the Round Table, and all that is best in men, when he committed adultery with Guenevere, which helped send Arthur into a Heroic B.S.O.D.? Why not the innocent and naive but faithful Determinator Perceval, who gets Arthur out of his funk with the Holy Grail?
The film is a heroic fantasy with a lot of dark fantasy elements. I guess Lancelot is all that is best in a man, but at the same time very much a man and capable of failure - he represents the whole of human existence. If Arthur is a shining example then Lancelot represents the urge for man to be better yet at the same time capable of weakness, he is human existence. I can see what you mean about Perceval - I always took him to be an expy for the common folk - never as glorious as lancelot but always persevering and surviving to the end, with a nobility of his own. Plus Perceval is the only survivor to live on and pass the tale down to the next generation, just as the small folk will pass the tale down.
Being the best KNIGHT isn't necessarily the same thing as being the best MAN. Knights embraced the concept of romantic love, not necessarily fidelity. He was also what is best in men... but not exclusively what is good in men. He had all the best qualities of mankind, in addition to a few fatal flaws.