Although the question is very difficult, Eisenhower comes to mind; his work on behalf of the Interstate Highway System - one of the greatest infrastructural achievements in human history - is praised even by those who are not in the habit of praising expensive federal projects (George Will, for instance, cited the IHS as being among the most successful of federal programs when queried by Al Franken). He supported the racial integration of our public schools despite the political risks inherent to such policies. And along with his clear denunciations of the very real threat posed to free countries by the Soviets and other totalitarian regimes, he also warned the nation of the problems that would arise from the increased entanglement between the government and private defense interests (it should be noted, of course, that he may have been somewhat naive about the Soviet threat himself during the early portion of his term, having signed a nuclear test ban treaty which the Russians promptly violated). This was, it seems, a man who had firm ideas of how things ought to be and who was willing to act on these ideas without much thought for partisan jockeying.

For me it would have to be our first george Washington when times were simple and not so much mud slinging want on and he helped create thsi nation after all.

If you have never seen the Historical Rankings of United States Presidents wikipedia entry, you should check it out. There is a sortable table that pulls together 12 scholarly studies that delved into this exact debate.

I happen to agree with 6 out of the 12 studies that Abraham Lincoln was the best US President. Lincoln successfully led his country through its greatest internal crisis, the Civil War, and ended slavery.

A neat place to check out if you are interested in Lincoln is the Union League in Philadelphia. Per the club's website:

"It was conceived as a patriotic social society whose purpose was to uphold the Constitution of the United States and support Abraham Lincoln as the President in his efforts to suppress the rebellion and reunite the country."

Abe Lincoln as mentioned above led the nation through the civil war and freed the slaves. Even his election was an incredible accomplishment.

I will say FDR because he got this country out of the Great Depression.