What makes Oedipus Rex a perfect tragedy?
As the audience reading/viewing this play, we are put in a position to fully empathize with Oedipus and his plight. This is one way to argue that Oedipus Rex is a perfect tragedy, which has adhered to Aristotle's definition of the tragic hero in its depiction of the story's protagonist.
One important factor in the success of the play as a tragedy is our sense that Oedipus is not at fault. He is acting reasonably, for the most part, and even courageously in his pursuit of the truth. Though he may pursue the truth with too much haste at times and too much passion, this endeavor is not marked by vice or by predictable folly.
Oedipus is acting on what he knows and his knowledge is limited.
The limits of human knowledge is, of course, central to the meaning of the play. This notion unites the audience with the protagonist in the same general plight. Connecting or relating to a universal theme is part of the power of tragedy. Sophocles' play explores a universal theme through its protagonist and so becomes something of a perfect tragedy.