3) and a tie between Jimmy Carter and George W. Bush (economic failings; caused from extremism of economic theories, just coming from opposite sides of the political spectrum).
1. Franklin Pierce. He put us on a path to Civil War and depression (the Panic of 1857 that began right after he left office).
2. James Buchanan. He never lifted a finger to prevent said war or mitigate said depression.
3. Richard Nixon. A criminal in foreign wars and in domestic affairs.
George W. Bush? He's up there (or perhaps "down there"), but he really wasn't the big problem in his Administration. Cheney was. He's more like Harding, or what some liberals (with some justification but not much) called Reagan: An amiable dunce. There wasn't a whole lot of malice to what he did. Contrast that with Cheney, and with Nixon, both of whom personified political malice in their respective generations.
By the way, for those of you who obviously skipped your American history classes in high school, Bush isnt even in the top 10, and all thumbs down prove me right ; )