As bad as Carter was, the country made an excellent recovery from his mistakes. That's the thing about being a bad leader -- your successor can make people think things weren't so bad. I don't know if Hoover was less competent than Carter, but his mistakes led to more lasting damage.
Right now it looks like Johnson and Johnson are the most controversial of the presidents. Present-day historians hate Andrew Johnson's bigoted views and lack of control, but a lot of the old "he saved the presidency" sentiment persists. Lyndon Johnson screwed up worse than Carter or Nixon, but some still value him for civil rights.
AJ #5.