Why is Ulysses Grant even on there? He prosecuted the Ku Klux Klan, enforced voting rights laws and rebuilt the Republican Party in the South. He wasn't terrible. Not saying he wasn't bad, but not top 6.
But I would say James Buchanan. He essentially let the Civil war happen.
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Yep, the Grant administration was actually much better than is generally known. George W. Pencilcock gets my vote for all time worst, but aside from the others references in this thread Polk deserves at least a [dis]honorable mention. He orchestrated what Grant later called "one the most unjust wars ever waged by a stronger against a weaker nation" (although we've outdone ourselves multiple times since then).