George Washington was the man who could have been king. Twice. The first time, he refused out right. The second time, he reluctantly accepted, though he went out of his way to instill a feeling that a president was merely a man, and not superior to the citizenry. Without Washington, I fully believe that this country would today be a monarchy. He set a standard of conduct that still hasn't fully eroded, after more than two centuries. He was simply the right man, at the right time.
I'd give Grover Cleveland the second spot. He's the only president who, while in office, actually increased the rights and liberties of the people.
Honorable mention goes to William Henry Harrison, who at least didn't take away any of our liberties.