Starbuck and Queequeg best represent "good" or goodness. Starbuck is the most beloved member of the crew and Queequeg is the most loving. These traits are demonstrated through direct exposition (regarding Starbuck: "'Thou art but too good a fellow, Starbuck,' says Ahab...") and through actions (regarding Queequeg).
Evidence for the crew's view of Starbuck can be found in many places in the novel. In particular, the first launch of the boats features a discussion of Starbuck and his reputation. Evidence of Queequeg's loving persona can be found in his first appearance on land as well as his heroic rescue of his fellow seaman from the head of a submerged whale.
This pair of figures, somewhat ironically considering Queequeg's paganism, also best represents the idea of religious orthodoxy and the humility which accompanies that spiritual posture.
Ahab stands in direct contrast to this religious orthodoxy. His goal is actually concerned with overthrowing an orthodox and humble relation to God. Ahab refuses to be subject to any will but his own. Enraged by the affront given to him by the whale and the God it represents, Ahab is bent on revenge at any cost.
“I am madness maddened!” he cries out to himself, alone in his cabin.
The reckless drive to achieve his goals without consideration for the well-being of his crew makes Ahab a representive of pride, evil, and hubris. Combined with his effort to overthrow all principles of primacy that would place a God above him, Ahab's lack of consideration for others makes him the best representative of evil in the novel.
(With this being said, the point should also be made that these figures, especially Ahab, are not drawn by Melville in such a way as to make them easily reducible to a single quality like "good" or "evil". The novel certainly presents some allegorical elements. Yet, the characters maintain a complexity and reality that resists reduction to a single trait.)