It's hard for me to say without putting some serious time and research into it. I won't say Carter because he at least had honor and integrity. The man was honest to a fault and that's worth something even if he didn't do a good job with the presidency.
Personally I'd probably have JFK near the top of the list of worst presidents. The only truly noteworthy thing he ever did was get shot. No meaningful legislation was passed in his tenure if I'm not mistaken, all he did was give some cool speeches and look pretty.
