Who are your top ten? I'm thinking that we could adopt Eurovision style points, being 1-8, 10 and 12 for the very worst. That way we could statistically put this question to the vote.
From tenth worst to the very worst, I award the following points:
1 - Herbert Hoover
2 - Richard Nixon
3 - Calvin Coolidge
4 - Ulysses S. Grant
5 - Franklin Pierce
6 - James Garfield
7 - Millard Fillmore
8 - Andrew Johnson
10 points - Warren G. Harding
Apart from the Veterans Bureau, the United States Shipping Board, the Office of Alien Property and fraud cases within the Department of the Interior and the Justice Department, Harding not only oversaw a hideously corrupt administration but as the President who brought in Prohibition, he hypocritically went on booze cruises to avoid the law which he oversaw. Clinton may have almost been impeached due to sexual relations with "that woman" but Harding had mistresses and even was paying child support for one of the children which came as a result.
Douze Point! - James Buchanan
Probably not a bad president if it wasn't for one giant glaring hole... secession.
In theory he could have brought the various factions together for a series of talks but he proceeded to do nothing. Even when his generals like Scott advised that troops be sent to protect government property, he did nothing.
I would think that usually the actions of a president usually determine how good or bad they are. Buchanan wins my list through inaction.
"I speak an infinite deal of nothing and I am not bound to please thee with my answers."