Ron Paul. I don't agree with his foreign policy, but I don't accept the neocon foreign policy that the other candidates advocate either, so that's a wash.
On domestic policy, he is right on. Since the New Deal we haven't had a President or a Congress that truly respected the Constitution when it comes to limiting federal power. Furthermore, the we've had a complicit federal court system that has gone along with myriads of intellectually dishonest, result-oriented decisions. It's time that we change that dynamic. 80 years of federal abuses of power are enough. If any of the other candidates became President, I think they'd largely support the status quo. It would be a third term of George W. Bush.
Since Paul isn't out to kiss anybody's ***, I like that he can be honest. He doesn't have to sit up there and pretend that Obama invented "big government." He certainly doesn't give Obama a pass, but he doesn't pretend that we had a responsible government before Obama. He can tell the truth about the drug war, corporate welfare, Social Security, accounting gimmickry in the budget, shell games being played with our currency, federal interference in public education, farm subsidies, etc. I'm a former partisan Republican who has become extremely cynical in the last 10 or so years. I hate being a disgruntled cynic, and nothing overcomes that like a politician I know in my heart to be telling me the truth.
It's also time for a political shake-up. The same groups have supported the same parties for decades. It has been unions, minorities, trial lawyers, gays, and God-haters for the Democrats, and corporate crooks, small businesses, and religious conservatives for the Republicans. It's getting old. A Paul nomination would likely lead to a political realignment. Many of the corporate establishment types who thrive on corporate welfare would abandon the GOP. However, many true small government, young libertarian types who have grown tired of the hypocrisy of a GOP that preaches small government but never follows through on it would likely move over to the GOP. Any change is good change.