The three worst presidents of the past 100 years, in reverse chronological order: Bush, Nixon, Hoover. Bush and Nixon shared contempt for the Constitution and a belief that they were above the law; Bush and Hoover shared an economic philosophy -- a religious faith in the power of laissez fair capitalism -- that led the country to the brink of ruin. Hoover and Nixon were gifted men, Nixon particularly so, while Bush is a bumbler. All three were Republicans, of course, but that's just a coincidence.
Bush clearly comes out worst here. He has no accomplishments as president, is not accomplished himself, and rivals Hoover in the economic damage he's done; when you take into account the damage Bush has done to the rule of law, and the political culture of the country, he crushes Hoover on the destructive index. Economies can be fixed, but once you start tugging on the threads of democracy and no one sews the holes you've left back up, well, that's trouble.
If Bush is the worst president of the past 100 years, that leaves about 110 years to comb through and come up with a contender for him in the battle to decide "Worst president in history." It's kind of a single elimination thing, like the NCAA tournament. My money's on Bush. I can't believe anyone from back then was handed so much, and walked away with so little.
I remember back in the day, when saying such a thing would instantly result in you being dismissed as a "Bush Basher." Ah, progress.