I don't have any single pick, but If I had to choose one, I would go with Washington. As the first President, everything that he did would establish a precedent for the future. And the fact that he did not try to wield as much power as he could have got our country (and Constitution) off on the right foot. In the early days after the Revolution, he could have maybe become King George. Instead, he choose to retire, and he had to be talked into running for President.
Reagan, in my mind, is a bit overrated and mythologized. I don't think that he was a terrible President, but conservatives who virtually worship the guy have a very selective memory. After all, under his watch, there were several tax increases, work visas for immigrants, and negotiations with the Soviet Union, the ultimate bad guy. (Not to mention the beginnings of huge budget deficits.) Is this the Reagan that they love?