Why has Oedipus Rex been called one of the most perfect dramatic plots ever devised?

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.