It’s too early to tell who rates as the worst president. Gallup polls and popularity contests do not tell it all. I simply do not trust them. Harry Truman, another famously unpopular sitting president reputation rose sharply as scholars began to appreciate his role in laying the foundations for America’s success in the Cold War.
There were however many failed and ineffectual pre-Civil War presidents with historical challenges (or in the cases of Grant and Harding, in the face of corruption and ineptitude inside their own administrations
Herbert Hoover is about at the 10 level from the bottom as he could never communicate as did FDR or Clinton. Nixon was politically gifted and actually accomplished many things until scandals brought him down.
Returning to the 1800’s we have Grant who I will always view with question. Graft and corruptions, but he had “good intentions” sort of sums it up. His being a drunk only added to the problem, But hey, we have present day politicians on both sides of the fence who are in this category.
OKAY! My VOTE is for Harrison for being so dumb to have an overlong inaugural address in a bitter D.C. winter. He died 30 days later. The others would be:
John Tyler staunch defender of slavery. Same for Millard Fillmore who allowed slavery to spread. Franklin Pierce actually expanded slave states and thus set the state for the Civil War which was a catastrophe for America.
For a “Do nothing award” it would go to James Buchanan. Harrison is exempt as he died before he could do nothing!
After the Civil War there would be Andrew Johnson. I think a winner in the ‘worst president’ realm.
More recently we have Warren Harding, who was totally ineffectual and made the present Wall Street ownership of politics in America look good.
Some of these characters made both Bush and Obama look good! 
(C) Herb Senft 2014