Since declaring the current president as "worst president ever" seems to be a habit for American people I decided to ignore presidents after 1990. The history will judge them too. This top meets the objectivity criteria According to Historical rankings of Presidents of the United States published by Wikipedia.
Nixon lied a whole nation that he never intervened in justice matters. He resigned from U.S. president position less than 9 months later to avoid impeachment for lying and obstructing justice in the Watergate Affair. He remained remembered for how he lied everybody without even blinking. More than that, he had ordered the secret bombing of Cambodia. He never admitted he did anything wrong during his term.
4. Andrew Johnson (1865 – 1869)
Harding administration was surrounded with moral turpitude. He had extramarital affairs with four women. Harding’s Secretary of the Interior Albert B Fall, was the first member of a presidential cabinet to go to prison for his participation in the Teapot Dome affair. When Harding got elected, he advantaged his friends (please see the Ohio Gang) to key political positions. These guys used their influence only to steal the country. Harding is famous to have said: “I have no trouble with my enemies, but my damn friends, my God-damned friends… they’re the ones that keep me walking the floor nights!”
2. James Buchanan (1857 – 1861)
Buchanan is the only president from Pennsylvania and the only president who stayed unmarried for entire life, and the last one born in 18th century. He was perceived by his contemporaries as a compromise between the two factions of the slavery question. In his first public speech as president, Buchanan said that the slavery issue was of “little practical importance” because the Supreme Court was about to settle it. Just couple of days after this speech US Supreme Court issued the Dred Scott decision in which it ruled that people of African origin, whether or not they were slaves, could never become U.S. citizens, and that US-Congress had no power to enforce slavery prohibition in federal states. Additionally, Buchanan’s presidency was also shaken by the Panic of 1857, an unexpected crisis in the U.S. economy. During his term, 7 slave states seceded, the Southern Confederacy came into being, all federal weapons and forts in the seceded territories were lost, and a 4th of all U.S. troops surrendered to Confederation soldiers. In 2006 historians said his failure to make a deal with pro-slavery South was the worst error of an U.S. president ever made.
1. Franklin Pierce (1853 – 1857)
Just couple of months before Pierce got elected as President, Pierce witnessed his son death in a train accident. He was on the edge of a nervous break-down. The most controversial event of Pierce’s presidency was the Kansas-Nebraska Act of 1854, which repealed what is known in history as the Missouri Compromise and established for dwellers the right to vote whether to permit slavery, in the name of popular sovereignty or rule of the people. The Act also caused a wave of outrage in the abolitionist North and spurred the creation of the Republican Party, a political faction that was organized as a prompt answer to the Act. Pierce is considered one of the least effective Presidents as well as a politician who was easily influenced. His main fault was the endemic lack of leadership he could provide to the nation. As of 2007, Pierce is the only elected president not to be renominated by a political party for a second candidacy as president.