Carter was the worst, for all of the reasons that Steve mentioned. Lyndon Baines Johnson was a close second. I have a book entitled A TEXAN LOOKS AT LYNDON by J. Evetts Haley, written before the election of 1964. LBJ had taken over as President when JFK was assinated in 1963, and he was up for re-election in the fall of 1964. I have loaned out my book to my brother-in-law, but the introduction by Haley went something like this, "I am fellow Texan, and a fellow Democrat, but I cannot let the American public go to the polls without telling them of what I know about Lyndon Johnson." The whole book deals with the underhandedness with which LBJ operated.
Here is a review of that book, as found on the internet:
Reviewer: Randall Ivey "Randall" (USA) - See all my reviews
Haley wrote this book (and published it himself) in 1964 basically as a campaign tract for Barry Goldwater. In the intervening years it has become a classic of its kind,a philippic, to use M.E. Bradford's term, tracing the illegitimate rise to power of Lyndon Baines Johnson. If you're politically naive, this book will grown hair on your chest. It's an unblinking, fearless portrait of Johnson's wheeling dealing and underhanded methods to achieve the power, prestige, and money he craved all his life. Haley names all the names and lays out facts and figures for the reader to make up his mind. And the reader winds up shaking his head in utter astonishment. The best part of the book is that detailing Johnson's eventual election to the U.S. Senate in a contest with former Gov. Coke Stevenson. The election was clearly Stevenson's, but through the machinations of George Parr, the notorious Duke of Duval County, the results were turned around in LBJ's favor. Investigators later found that among those voting in the primary were people who didn't live in the county anymore and people who weren't alive at all. But the results stood.(An interesting and amusing aside: when Haley ran for Texas governor in 1956, he approached Parr and said, "I'm Evetts Haley. I'm running for governor, and if I win, it will be my privilege to put you in jail." Parr's reply: "I believe you will." Parr, the Artful Dodger of Texas politics for years, eventually killed himself.)
At times the book grows tiresome, especially in the Bobby Baker and Billie Sol Estes scandals, where Haley turns a virtual torrent of names and numbers on the reader as to be sometimes confusing. But slog through those sections to get to the excellent chapter where LBJ wields an iron hand as Senate Majority Leader and maneuvers himself into the Vice Presidency. It is chilling.
Haley suffered considerable damage to his reputation as a result of this book, but he never backed down, and now time seems to be bearing out the truth of his claims, which have also been verified in such later works as the Robert Caro books. This is an indispensable lesson in this Presidential election year. Read it and weep.
Anyway those two, Carter and Lyndon Johnson, get my votes as the two worst presidents in American history. Clinton was awfully awful, also, but the other two were worse.