Why has Oedipus Rex been called one of the most perfect dramatic plots ever devised?
Oedipus is the quintessential tragic hero in the quintessential tragic arc. He is, essentially, a good man who suffers from a major flaw (pride, or hubris); this flaw leads to an error in judgment, and this mistake leads to a reversal in his fortune and his ultimate downfall. Oedipus wants to be a good king—he seems to legitimately care about his subjects and wants to help and protect them. However, when he learned from the oracle (prior to the play's beginning) that he was destined to kill his father and marry his mother, he decided not to return to Corinth, where the people he thought were his birth parents lived. He proudly thought that he could avoid the prophecy—the will of the gods—but no mortal can thwart the gods. Oedipus learns the hard way that his pride heralds his downfall, when he discovers that the prophecy has come true.
Oedipus Rex is admired for its plot because of two closely related qualities: the peripeteia and the irony. "Peripeteia" means reversal, and it is hard to image a more complete reversal than occurs in this play. Oedipus is a king, and wants to do what is right by his people, and, specifically, to cure the plague ravaging them. What does he learn when he investigates? That the cause of the plague is…himself! That he killed his father and married his mother, precisely because of an attempt to dodge a prophecy that he would do that very thing! That, combined with his wails of agony when he realizes that his whole life is the reverse of what he planned, are exquisite.