It's hard for me to argue with Prof. Foner's assertion that George Bush is the worst President of all time; his competition can be mainly separated into qualified-disasters-with-important-accomplishments-on-the-side, like LBJ and Nixon, or Civil War Era boobs who presided in the White House at a time when the power of their office was weaker, and the U.S. was nothing more than a regional power (Pierce, Andrew Johnson, Buchanan). Bush lacks the very real accomplishments of Johnson in the domestic arena (Civil Rights and Voting Rights Acts, Medicare, etc.) or Nixon in foreign policy (reproachement with China, detente with the Soviet Union), so he has much more in common with the latter group of men, with the important difference being the scale of American global power when he assumed office. Buchanan and Pierce may have been slow to react to Confederate treason, but at least they didn't jeapordize America's role as a superpower by their actions.
In short, Bush managed to hit the trifecta: our economy stagnated, our relations with the rest of the world worsened, and our nation is weaker than it was before he became President. Those points are important, since they go to why we elect people to lead us in the first place. We elect our Presidents in the hope that they will make the country a better place, not to follow some ideology or to act as a national role model. While the rankings generally reflect the political leanings of historians, which currently tend to skew left-of-center, there is little doubt about the men at the very top; it's Washington, Lincoln and FDR, and whatever shortcomings they may have had concerning African-Americans or women, or whether their views would be considered palatable today, is irrelevant to the totality of their ranking. A liberal historian might judge Jimmy Carter more kindly than William Howard Taft, or might rank Ronald Reagan worse than he deserves, but the true greats are not debated.
And the same thing is true at the bottom. It isn't just that James Buchanan was a pro-South sympathizer who hated blacks. The same thing could be said about almost all of the Presidents before Lincoln, and most of the Presidents that followed him up until the end of WWII. Bush, like Buchanan and Pierce, is a disaster because he followed policies that proved cancerous to the nation.