Aristotle was all about Oedipus the King. He probably drew little hearts all over his copy of the script and slept with it under his pillow. Aristotle cites Oedipus constantly in his how-to manual for tragedy, Poetics. The play meets nearly all of the tragic criteria that Aristotle lays out in this famous book.
For one, the story is of significant size; its events have universal ramifications. The characters are of proper stature for tragedy. Oedipus is a real live hero with mostly good intentions, making his downfall all the more painful to behold. The hero is made tragic by his hamartia, which is often called a tragic flaw, but is more accurately translated as an error made in ignorance or a missing of the mark. Check out Oedipus's "Character Analysis" for an extensive discussion of his hamartia.
The thing that really makes Aristotle all googly-eyed over Oedipus is its expert plot construction. The play has a distinct, beginning, middle and end, linked by logical cause and effect. Each event brings the next. Most importantly, everything is caused by the actions of our tragic hero, Oedipus. Though he acts in ignorance, it was Oedipus who killed his father and slept with his mother. It is Oedipus's relentless investigation that reveals the horrible truth. It is Oedipus who blinds himself and begs Creon to banish him. The fact that most of these actions were the result of good intentions is what makes Oedipus's fate truly tragic.
The play also has great examples of some of Aristotle's favorite plot devices: peripeteia, anagnorisis, and catastrophe. Peripeteia means a reversal of intention or a turning point. In Oedipus this happens when the Messenger shows up from Corinth. The man tries to ease the King's mind by telling him that he's not really Polybos's son. Though the Messenger intends only good things with this information, it ends up being the thing that drives Oedipus toward his horrible fate. The wicked irony of this turning point makes it a pitch-perfect peripeteia.
Anagnorisis means a recognition. It's where the tragic hero suddenly realizes the terrible truth. In Oedipus the King it happens when the combined testimonies of the Corinthian Messenger and the Theban Shepherd make Oedipus realize that he's unwittingly fulfilled the prophecy he's struggled to avoid. One of the things that makes Aristotle think the play is so great is that the anagnorisis is directly caused by the peripeteia. The words of the Messenger are what cause Oedipus to summon the Shepherd. The two plot devices are linked by a terrible but inevitable logic.
Just as the peripeteia directly leads to the anagnorisis, the anagnorisis directly leads to the catastrophe, or the terrible suffering. When the truth is revealed, Jocasta hangs herself, Oedipus stabs himself in the eyes, and begs to be banished. All these things add up to make Oedipus the King the gold standard of tragedy, at least according to its biggest fan, Aristotle.