None really stand out as bad, but I vote Truman. FDR was the biggest FF ever. No president even comes close to FDR's greatness.
Why Truman? Unless you have an issue with him dropping the two bombs, I can't find a reason to declare him the worst. I dislike him because of Korea and the Truman Doctrine, but if you're going to go down that route more people died under Johnson's watch in Vietnam than did under Truman in Korea. (Well, the numbers might be close but Johnson f'ed up far worse in Vietnam than did Truman) Of course, maybe you hold Truman responsible for every Cold War American casualty, but I don't see that as being entirely fair.
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