Without a doubt the most over rated is Ronald Reagan, who is regarded as some kind of paragon when his major accomplishments were undermining the financial sector through deregulation, violating Congressional sanctions on Iran and funding terrorists in Nicaragua to undermine a legitimately elected government. Ronald Reagan created the savings and loan meltdown that was blamed on H. W. Bush, and is credited for winning the Cold War because he was in office when the Soviet Union imploded although that path was inevitable after Truman, Eisenhower and Kennedy. Reagan was elected by attacking Carter over the hostage crisis even while he had made a deal with Iran to keep them until after the election, in exchange for an arms deal that eventually sent Oliver North to prison.
Most under rated was Lyndon Baines Johnson, who managed to get John Kennedy's civil rights agenda through Congress despite hysterical opposition from the Dixiecrats and his awareness that the racists would vote Republican "for the next hundred years." That and his social welfare legislation are taken for granted and credited to others while he is blamed for the failure of the Viet Nam War. Definitely the most skilled legislator and negotiator ever to sit in the Oval office.
Understand this is not best and worst. Those would be Abraham Lincoln and James Buchanan, respectively, but history has judged them both fairly accurately.