Who were the greatest presidents?

I heard Joseph Ellis the other day considering who were the greatest American presidents. He chose Washington after saying that most historians agreed that Washington, Lincoln and Franklin Delano Roosevelt were the three greatest presidents.

Those three faced the greatest challenges -- the creation of the office (and of course leading the troops in the Revolutionary War), the Civil War, and the Great Depression and World War II respectively. It is of course possible that the process of rising to such a challenge makes a man great.

It might be that historians distinguish between greatness in a man and greatness in a president. Were Washington, Lincoln and FDR really greater men than Jefferson, Grant, Wilson or Eisenhower, or did they simply face and survive greater challenges during their presidencies?

Somehow one would want to point to great men as role models, and in that sense one would wish to identify greatness with character and virtue. Indeed, might the greatest president not be the one who most avoided leading the nation into peril?