This doesn't seem a bad choice:
Galahad. I actually had to look him up when he beat evil Lancelot in "The Librarians". Actually, in the original Arthurian tales, Lancelot was kind of a tool. Galahad was cool. He is thought to be a gift to humanity as the greatest knight, but his real claim to fame seems not to be his skill with a sword but his purity, leading him to find the holy grail.
You could do this whole thread with a different question that would be very, very hard to answer: who is the greatest swordsman of Game of Thrones, taking into account their best years may NOT coincide with their nearest competition.
Not sure how I'd answer. Jamie Lannister, with his hand gets a big vote. But he sees in the Knight of Flowers, himself in his youth.
Fox12 said:
Arthur was supposed to be very, very good but due to the power of Excalibur. There have been many versions of the tales and typically, Lancelot was better. After some searching after seeing that episode of "the Librarians", arguably, the legend has it that Lancelot's son Galahad was even better. http://en.wikipedia.org/wiki/Galahad