WaPo's Outlook section has the scorecard:
Eric Foner: Oh yeah, definitely. Disdain for Constitution and rule of law puts him over the top. Or under the bottom, as the case may be.
Douglas Brinkley: I dunno, probably. Depends on Iraq, which is not promising. But at least Bush is honest(!) and his administration isn't corrupt(!!!) - apparently Brinkley's not a big fan of the Constitution.
Michael Lind: He's only fifth worst! Woohoo! It's all about the unnecessary wars, and Iraq not as catastrophic as Civil War or War Of 1812. Warrantless wiretapping only used against suspected terrorists, and not against political opponents as with Nixon. (Dude, are you sure about that?)
Vince Cannato: Time will tell - if things pan out well in the Middle East, history might even remember Bush as mediocre! Not-liberal does not equal bad; accusations of trashing the Constitution are just "hyperbole," Bush understands "tough tradeoffs between security and privacy." Guess who Cannato worked as a speechwriter for in 2001.
I think there are actually two broad dimensions on which awfulness (or greatness) can be measured: competency and outcomes. I believe Warren G. Harding was one of the most incompetent and corrupt presidents of all time, but nothing especially terrible happened on his watch, even if he did help set the stage for the Depression. On the other hand, the incompetence of Madison and Buchanan essentially maximized the damage of two wars that might have been preventable or containable.
Bush has managed to score very low on both axes: He's run a series of businesses into the ground, and he employs that same shrewd managerial acumen to the job of running our country, with predictably similar results. That alone would be enough to rank him with the likes of Harding, Coolidge, and Hoover in terms of competence. But throw in a disastrous, unnecessary war in which Iraq and the Constitution have sustained far more damage than al Qaeda; throw in passive, uninterested responses to imminent catastrophes on 9/11 and in New Orleans, and you've got enough to catapult Bush way past Nixon and Madison and Buchanan. The fact is, Bush has simply screwed up in more different directions simultaneously than any other president in American history. I think Nixon is the only one who comes close, but his crimes are mitigated by the fact that A) He was actually a pretty competent president who achieved some positive things, and B) He inherited his stupid, unnecessary war.
So yeah, unless Bush's magical thinking magically pays off in Iraq, and it suddenly blossoms into a model democracy and ushers in a magical new Golden Age in the Middle East, you can put me down for "Worst Ever" in the Bush All-Time Presidential Ranking pool. I'll be back to collect my winnings in 2050 - in Euros, please.