My favorites from college are: early dialogues of Plato (the most Socratic / least Platonic -- The Apology is a good start), Immanuel Kant's Critique of Pure Re

My favorites from college are: early dialogues of Plato (the most Socratic / least Platonic -- The Apology is a good start), Immanuel Kant's Critique of Pure Reason (on how deeply the very ideas about how we perceive things affect our understanding), and Aristotle's Nico. Ethics (the importance of habits, moderation, and the idea of the good).