Ron Paul gave a great speech on the House floor warning of the dangers of a police state in the U.S. (I think it was in 2003). Needless to say, that rubbed a lot of Republican fur the wrong way. He has never carried water for the Bush/ blue-blood wing of the party. I really like this guy, but the RNC machine will no doubt close ranks on him.
Update: If you truly value the Constitution and the values upon which our great nation was founded, Ron Paul is probably the best of all the candidates- on either side. That said, he stands about as much chance of being Pres as I do.