I'd have to go with Ol' Franky Pierce as the worst president.
Here's some fun facts:
After losing the Democratic nomination, Pierce reportedly quipped "there's nothing left to do but get drunk."
He reportedly once ran over an elderly woman while driving a carriage intoxicated.
Harriet Beecher Stowe referred to him as "the archtraitor."
During the Civil War, Pierce declared support for the Confederacy and loudly denounced Abraham Lincoln, whom he wholly blamed for the war.
Pro-slavery Border Ruffians, mostly from Missouri, illegally voted in a government that Pierce recognized.
During his first years in Washington, as a young Congressman, he kept company with a hard-drinking crowd, showing his manliness by competing with his companions in drinking capacity.
Soon after Pierce's inauguration, one of the members of his party wrote of the President: "I deeply, deeply deplore his habits, he drinks deep . . . A great mistake was made in putting him in at all."
en.wikipedia.org
www.healthguidance.org
There's more, but basically he was a depressed, physically-ill, raging alcoholic who strongly supported slavery and recognized illegal governments within the U.S.