Hard to choose. I guess George W. Bush was the worst -- although for American leftists like me, I think he was a godsend. He implemented conservative Republican principles in a way that made them disgusting to almost everyone with a brain.
The more dangerous screw-up as a president, I think, was Ronald Reagan, because he remained popular while doing terrible things to this country and the world. Maybe because his brain was being rotted by Alzheimer's disease, Reagan funded future leaders of Al-Qaeda in Afghanistan, on the grounds they were "religious freedom fighters." He presided over the worst economic recession that the United States had suffered since the 1930s, with the average unemployment rate rising above 10% in 1982 and 1983, while the jobless rate for young black men and women -- "last hired, first fired" -- rose to approximately 50%.
Reagan also promised that he would "never" negotiate with the Islamic hardliners in Iran, whom he labeled as "terrorists," but in the famous "Iran-Contra" scandal, his national security people did exactly that.
Reagan had campaigned for office in 1980 by promising to balance the federal budget, while also slashing tax rates and increasing defense funding. His rival for the nomination and future vice president, George Herbert Walker Bush, rightly labeled this "voodoo economics," and George H.W. Bush was correct. Over Reagan's 8 years in office, the government gave a $1.7 trillion tax cut to American business, and the size of the US national debt tripled, as the government's yearly deficit soared out of control.
Under Reagan's leadership, the US in 1985 dispatched Donald Rumsfeld as an unofficial ambassador to Saddam Hussein's brutal dictatorship in Iraq, where Rumsfeld helped to establish friendly relations between the US and Saddam's regime. Over the next few years, Reagan's Commerce Department OK'd the sale of previously restrictured US goods with potential military applications to Saddam's government - including "denatured" anthrax spores and other potential germ warfare agents, and supposedly demilitarized HU-1 helicopters, which Iraq promised to use for civilian purposes, but which Saddam's government probably used in poison gas attacks on Kurdish Iraqis. Reagan in this way therefore helped Saddam Hussein to grow into a Frankenstein monster, which George W. Bush in 2003 then helped to slay -- at the expense of throwing Iraq into an apparently permanent religious civil war. Nice going, Ronnie!
In Central America, in the guise of supporting "democracy" and "freedom" and fighting against Communism, the Reagan administration provided support to the government of El Salvador, at a time when the Salvadoran generals were dispatching "death squads" to torture and murder political dissenters.
Back at home, Reagan by breaking the 1981 strike of the Professional Air Traffic Controller's union (PATCO) signaled to US corporations that they would have basically free rein to force bad contracts on unionized workers. The Reagan administration also made a mockery of federal enforcement of civil rights law, and put a conservative, anti-environmental lawyer, Anne Gorsuch Burford, in charge of the US Environmental Protection Agency.
Yet while Reagan was doing all this crap, paid ads on TV paid for by giant corporations declared that it was "Morning in America Again," and American broadcast media owned by right-wing corporations hailed Reagan as "the great communicator."
So I'd say that Reagan was even worse than George W. Bush, even if the conservative spin machine and Reagan's own folksy manner both caused him to be fairly popular with voters. Reagan was not only inept and evil, but also a great liar and spin artist. Poor George W. Bush was inept and evil, but a lousy spin artist. He therefore did less damage.
-- democratic socialist