Who was the worst president in history?

The same five names come up all the time: James Buchanan, Warren G. Harding, Andrew Johnson, Richard M. Nixon, and George W. Bush.

James Buchanan served from March 4, 1857 to March 4, 1861. Historians credit his detachment from the slavery question as one cause of the American Civil War. Andrew Johnson served from April 15, 1865 to March 4, 1869, and scholars assert that he fumbled the Reconstruction effort. Warren G. Harding served from March 4, 1921 to August 2, 1923, and his short administration was rocked by the Teapot Dome corruption scandal. Richard M. Nixon served from January 20, 1969 to August 9, 1974, and needs no further introduction. Neither does current president George W. Bush, inaugurated January 20, 2001.

No student of history, I will leave the answer to the reader. Recall that George Bush has yet to complete his term, and that while the administrations of Buchanan and Johnson are described with words such as "failure" and "ineffective," those of Harding and Nixon are described with words like "corruption," even "treason." Failure and ineffectiveness are passive crimes. Corruption is the active violation of a sworn oath.

But then again Nixon's administration may have been more moderate than younger readers will know:

As President, Nixon imposed wage and price controls, indexed Social Security for inflation, and created Supplemental Security Income (SSI). The number of pages added to the Federal Register each year doubled under Nixon. He eradicated the last remnants of the gold standard. Nixon created the Environmental Protection Agency (EPA) and Occupational Safety and Health Administration (OSHA), promoted the Legacy of parks program and implemented the Philadelphia Plan, the first significant federal affirmative action program, and dramatically improved salaries for U.S. federal employees worldwide. As a party leader, Nixon helped build the Republican Party (GOP), but he ran his 1972 campaign separately from the party, which perhaps helped the GOP escape some of the damage from Watergate. The Nixon White House was the first to organize a daily press event and daily message for the media, a practice that all subsequent staffs have performed.

Reason has an excellent survey on similar matters here.