Which four pilgrims are represented as being the most virtuous members of the group?
While I think the last two noble characters are up to discussion (jdellinger gave some fine examples), the two most virtuous members as described in the prologue are without question the parson and his brother the plowman. The parson is the only clergy member on the trip who does not indulge in money, food, drink, flesh, etc., and because of that is poor and must work harder. His brother, the plowman, is also described as being an extremely hard working laborer who prefers to earn his living honestly.
While Chaucer portrays no character as wholly good or wholly evil, there are some whom he respects more than others:
The Knight: "He graciously accepts the shortest straw with chivalrous courtesy."
The Squire: "There is little mystery, however, as regards the theme of this narration. It strongly promises to deal with wonders, constancy in love, and virtuous character. Ideal love will no doubt triumph in the end."
The Franklin: "The Franklin's Tale concentrates on the relationship between husbands, wives, and lovers, exposing the vices and virtues of men and women." "There is nothing crass about Aurelius and Dorigen, for although both of them err, all are shown in the end to be capable of great honor, loyalty, and generousity. The sanctity of marriage is upheld and respected in The Franklin's Tale."
The Second Nun: "The tale itself is exactly what it appears to be, the life of a saint." " is womanly but not weak; indeed, she has none of the shortcomings of any of the other women characters in the tales. There is no question that she is presented to be imitated."