The Theban Trilogy by Sophocles. Those are the 3 plays: Oedipus Rex, Oedipus at Colonus, and Antigone. They really got at the heart of the human condition, and Oedipus at Colonus, which was written last decades after the first 2, shows man to be the pivotal point of the world, the link between the sacred and the profane. It changed my whole way of thinking. It brought into focus how precious humanity is. To paraphrase Carl Sagan, we are the universe knowing itself.