One thing I like about Arthurian literature is that the same story is told over and over again; one might even say that the creative emphasis is on delivery rather than content. I was fortunate to be reminded of this today in a used book store when I picked up La Mort le Roi Artu (in translation), not to be confused with the Alliterative Morte Arthure, the Stanzaic Morte Arthur, or Malory's Le Morte d'Arthur, the most well-known of the four.
In all these iterations of the fall of Arthur, one of the main themes is the question of what makes a good knight. Ideas of faithful love, honor, and bravery rise as answers to this question, as well as skill in riding and fighting, the literal meaning of 'chivalry'. If you ask who the Best Knight of the Round Table, many people might say Lancelot, who is essentially never defeated in combat, but whose affair with Guinevere is the largest catalyst of the death of Arthur and the destruction of the flower of chivalry. Another common answer is Galahad, who is even more unbeatable and perfect than his father Lancelot, but he is almost more of a priest than a knight, and he is barely 'of the Round Table' because his only quest is the grail and he fights more Round Table knights than he cooperates with. Tristram is another essentially unbeatable knight, but he shares all of Lancelot's flaws and fewer of his virtues.
If I was pressed to name the Best Knight of the Round Table, I would probably go with Percival or Bors, but I think the most interesting answer is Gawain. In most Arthurian tales, Gawain is often defeated and commits many sins, but he nevertheless remains one of the most esteemed and influential knights of the Round Table. There are many sides to this conversation, but for now I propose that Gawain is a good knight because he admits his flaws and shows contrition. This passage at the beginning of La Mort le Roi Artu gives a good example:
"The king [Arthur] had heard the rumor that Gawain had killed several [knights], and he summoned him before him and said:
'Gawain, I order you, by the oath you swore when I knighted you, to answer the question I am going to ask you.'
'My Lord,' replied Sir Gawain, 'since you have asked me in that manner I shall not fail in any way to tell you, even if it brought me shame as great as ever befell a knight of your court.'
'I want to ask you,' said the king, 'how many knights you think you killed, by your own hand, on this quest.' Sir Gawain thought for a moment and the king said again: 'By my oath, I want to know, because there are people who are saying that you have killed a very large number.'
'My Lord,' said Sir Gawain, 'you obviously wish to be certain of my great misfortune, and I shall tell you, because I see that I must. I can tell you in truth that I killed eighteen by my own hand, not because I was a better knight than any of the others, but since misfortune affected me more than any of my companions. Indeed, it did not come about through my chivalry, but through my sin. You have made me reveal my shame.'
'Certainly, my nephew,' said the king, 'that was truly great misfortune, and I am well aware that it happened through your sin. Nevertheless, tell me whether you believe you killed King Baudemagus.'
'My Lord,' he said, 'I definitely did kill him - and I have never done anything that I regret so much as that.'
'Indeed, my nephew,' said the king, 'if you have regrets about that it is not surprising; because, may God help me, I regret it too. My court has lost more in him than in the four best knights who died on the quest.'
Gawain is clearly a flawed knight in this scene, but contrast his admission of weakness with Lancelot's behavior during a fight with a young Gareth in Le Morte d'Arthur:
"[Gareth] fought more like a giant than a knight; and his fighting was so passing durable and passing perilous, for Sir Lancelot had so much ado with him that he dreaded himself to be shamed, and said, 'Beaumains, fight not so sore! Your quarrel and mine is not so great but we may soon leave off.'"
Shame is mentioned in both passages; Gawain publicly accepts his shame and shows contrition, while Lancelot fights to preserve his image. Many years later, after being caught with Guinevere, Lancelot continues to fight rather than admit fault, to the point of killing Gareth (who loved Lancelot more than his own brothers) and destroying the Round Table. A full discussion of knightly shame would require many more sources; for example, Gawain's ability to confess sin and accept the consequences at the cost of reputation is a major part of Sir Gawain and the Green Knight.
Just as Gawain's moral weakness is redeemed through confession, his relative weakness in combat leads him to be a more good knight, while not a better knight, than his stronger compatriots. There is an idea in chivalric stories that right makes might, i.e. whoever is on the side of justice will prevail in combat (which is why trial by combat is valid). Because of this, one might say that since Lancelot always wins battles, he must be the most morally upright knight. On the other hand, Lancelot fights just as well when killing unarmed knights to save Guinevere from the consequences of his actions as he does on any of his nobler adventures, and there are plenty of tales with evil knights who are strong enough to defeat all but the best from the Round Table. Therefore, I would say that an often-defeated knight such as Gawain shows more bravery in going out questing than an undefeated knight such as Lancelot or Tristram; it takes more fortitude to lose a fight and suffer for doing good than it does to go from victory to victory.
Anyone, then, with the necessary equipment and skill in fighting might be good at being a knight, but as Gawain shows us, it actually takes weakness to be a Good Knight.