Immanuel Kant
I've always enjoyed studying the works of Kant. He is by far my favorite philosopher and echos most of the principles I aspire to live by.

Critique of Pure Reason (mirror)
What is Enlightenment? (mirror)
Metaphysical Elements of Ethics (mirror)
Critique of Practical Reason (mirror)
Critique of Judgement (mirror)
Science of Right (mirror)
Introduction to the Metaphysic of Morals (mirror)
Fundamental Principles of the Metaphysic of Morals (mirror)