I don't know how you can choose just one as the best ever, but it sure as hell would never be Reagan, no matter what the fanboys tell you.
My votes are for Jefferson, Lincoln, Wilson, Hoover (j/k!), and FDR.
At the same time, Taft did the most important things after he was president. Sure, Carter builds houses with his free time, but Taft followed up his bathtub stuffing administration by being Chief Justice.