Carter was hardly a good President, but considering that he's frequently cited as an example of one of the worst in history of whatever I think he's a good contender for this title.
Bonus: Most overrated President - Clinton, Clinton, Clinton. It never fails to amaze me how the Clinton administration seems to exist in the popular imagination as a collection of savvy, pragmatic pros with unmatched political skills instead of the scandal-plagued, half-competent near disaster that it really was.
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Especially because, compared to Clinton, Obama looks like a progressive hero.