Statistician Nate Silver of Five Thirty Eight and The New York Times created a composite list in January 2013 of previous presidential rankings by scholars. This is as definitive a ranking as we have at this time. George W. Bush is 38th of 43. The surveys Silver used were conducted between 2008 and 2011, when Bush was still getting a free pass for the 9/11 attacks.
The latest and most convincing accusation leveled at George W. Bush for allowing the 9/11 attacks comes from the lips, on camera, of ex-CIA Chief George Tenet and ex-chief of CIA counterterrorism, Cofer Black. The latter says, "How is it that you could warn senior people so many times and nothing actually happened?" Tenet, Black, and others say more just as damaging, in The Spymasters: CIA in the Crosshairs currently on Showtime. You can read about the documentary's revelations in writer-producer Chris Whipple's article in Politico.
Whipple corroborates recent articles by Elizabeth Drew in The New York Review of Books and Peter Beinart in The Atlantic. "Bush was insufficiently vigilant. The evidence is overwhelming," Beinart concludes. This would have been characteristic of W in the context of his failure to heed warnings about Hurricane Katrina and his taking more vacation time, by far, than any other modern president.
Here's the rap sheet for the bottom six presidents with links to and quotes from their Wikipedia pages. Judge for yourself who is the worst, considering Bush's added responsibility for 9/11.
William Henry Harrison (1773 - 1841) was the 9th president and 40th in rank. He "died on his 32nd day in office... serving the shortest tenure in United States presidential history."
Republican Warren Harding (1865 - 1923) was 29th president and ranked 41st. When he died about half way through his first term, he was "one of the most popular presidents in history," but soon there were revelations about his adultery and the Teapot Dome Scandal, which involved bribery and Navy oil. Plus, there was Harding's own opinion of his suitability. "I'm not fit for this office and never should have been here," he confided to a friend. "My God, this is a hell of a place for a man like me to be." Still, the crimes are petty, so Harding and Harrison belong on the same shelf in the basement by default.
The other three below W were pretty bad however. Andrew Johnson (1808 -1875) was the 17th President and he is ranked 39th. A pro-Union Southerner, Johnson opposed an effective Reconstruction and the Fourteenth Amendment, federally guaranteeing former slaves' rights, and ran into the wrath of congressional abolitionists. In impeachment proceedings, he came within a single vote of being removed from office. Johnson could have qualified as the worst, but he was kept from doing his worst by a veto-proof congress.
Franklin Pierce (1804 -1869) was 14th president and ranked 42nd. "His polarizing actions in championing and signing the Kansas-Nebraska Act and enforcing the Fugitive Slave Act failed to stem intersectional conflict, setting the stage for Southern secession."
Down at the very bottom is James Buchanan (1791 -1868), the 15th President. He won in a three-man race and was a Northerner with Southern sympathies. "Historians in both 2006 and 2009 voted his failure to deal with secession the worst presidential mistake ever made." If that failure actually caused the war, then he would beat Bush.
But Buchanan and Pierce are historical road kill, just two of a long list of contributing factors, like the Kansas-Nebraska Act and the Fugitive Slave Act, stretching back to the founding of the republic. The best mind to occupy the White House and Buchanan's successor, Abraham Lincoln, tried desperately to stop that war in his first months in office and failed. Anything Buchanan could have done would have been just forestalling the inevitable.
However, George W. Bush's was a pointless "war of choice," making that an even bigger blunder than allowing the 9/11 attacks. Plus, he contributed to the biggest economic collapse since the Great Depression and converted a budget surplus into a 1.2 trillion dollar deficit. Plus, there was Katrina, torture, no-bid billion dollar contracts for cronies, anti-gay bigotry in his reelection campaign, etc. Readers can add favorites.