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Thomas Aquinas, of course! Were it not for him, Aristotle (The Philosopher) would likely have been lost to history.... or at least lost to western civilization. He teaches very pure reason, and you can stop reading once he has proved the Trinity, but that would be your loss.

As far as modern philosophers go, my favorite has to be Peter Kreeft. Check him out.

This lecture series will take you thru Philosphy, and you might even end up loving Peter Kreeft by the end of it.

I'm just glad a couple of us have a favorite philosopher, I'll listen to the vid, one of my current favorites is Peter Adamson at Kings College, "the history of philosophy without any gaps", very entertaining too.
http://www.kcl.ac.uk/innovation/action/ArtsHums/hist-phil.aspx

Descartes! By a long shot.

I do love the simplicity of thinking Socrates actually did roam the halls of the marketplace asking the basic questions about the nature of reality.
Today, the philosophers have all specialized, written papers nobody reads and stay confined in a box of academia-useless to society.
I believe Aristotle wrote way more than the rules of philosophical engagement and his real works are hidden away, but he was and in my mind still is the greatest.
After Aristotle religion took hold of Philosophy and here we are...
Thank god for Molineux attempting to keep Philosophy alive. My least fav is Hidegger, like the physicists of today: make everything too complicated so people feel too stupid to get involved.
Pure philosophy should promote the most basic of questions, what is my place in the universe, is there a god and why doesn't anyone care anymore.. Thanks for asking...