Coolidge.
The reason FDR, Lincoln, and Washington are always considered the greatest is because people are lazy or want to pick who they think everyone else is going to pick.
Coolidge was really one of the forgotten presidents until recently. I believe Glenn Beck has praised Coolidge on several occasions, and the Bill Bennett American history textbook (America: The Last Best Hope, which I recommend everyone interested in U.S. history read) treats Coolidge very well.
My favorite Coolidge anecdote concerns his refusal to speak unnecessarily. When a woman approached him at a dinner, she informed the president, “I made a bet with my friends that I could get you to say more than two words.” Coolidge’s reply? “You lose.”