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