Must reads I would recommend Plato's Meno, the text in which he went from being an author of manners and committing suicide to the skeptic he had been until then. Or The Sophist, where he dismisses almost everything that he previously thought, including the positive assessment of Socrates.
The second book that attracts me the most is Categories by Aristotle. I had the opportunity to have an old teacher of mine use it as a text to teach us how to translate Greek. It was an exciting experience, although it emptied the classroom.
A third text is Kant's Critique of Judgment, where he tries to correct himself. It is a text in which I always discover something new and it makes me feel quite an idiot. It is good for me to remember that I am not as smart as I am sometimes led to believe.