Galahadachieves it
Faultless from birth, and built for exactly this. He sees what tongue cannot tell and asks to die.
The Grail
Who reaches it, and who is stopped
The quest is not an adventure with obstacles. In the French tradition it is an assessment, and the assessment is the point: a hundred and fifty knights ride out, and the ones who come back have been told exactly what they are.
Percivalachieves it
The Grail's original hero, demoted to a companion once Galahad exists. In Wolfram he is the one who succeeds, and the difference is the whole argument.
Borsachieves it
The only one who comes home. Not a virgin, not faultless — the man who made a hard choice and paid for it, and went back to a doomed court and stayed.
Lancelotfails
Gets further than anyone else who fails: as far as the chamber door, where he is struck down and lies twenty-four days, one for each year of the love.
Gawainfails
Told in a paragraph that he will find nothing, because he has not confessed in years and does not intend to. He accepts it and rides home, killing eighteen companions on the way.
Percival's sisterachieves it
Gives all her blood to cure a stranger and dies of it. Her body reaches the Grail city ahead of the knights. The text never gives her a name.
Anfortasis healed by it
Not a quester but the reason for the quest. In Wolfram he is cured when somebody finally asks what is wrong with him.
Percival’s sister
She guides the three knights, explains the ship and the sword, and cuts off her hair to weave a new belt for it. At a castle whose custom demands a dish of blood from every gentlewoman who passes, she volunteers, is bled until she dies, and asks to be set adrift in a boat after them. Her body reaches the Grail city ahead of the knights and is buried there.
She gives more to the quest than anyone except Galahad, and the text never gives her a name of her own. Whether that is the sharpest thing in the cycle or its worst blind spot is a question worth holding open.