It might be the fact that the first thing Team Bush did was accuse the outgoing Clinton staff of vandalizing the White House. A story that turned out to be false, of course. The fact that the very first thing they did upon taking power was to lie, that's a pretty good legacy.
The Iraq war is an easy answer, of course. Especially with everything going so well.
There's also the fact that Bush once vowed to track down bin Laden "Dead or alive", but six years later, he still has no plan for doing so.
But for me, I think the Bush legacy will be summed up by his first Attorney General, John Ashcroft. Mainly because Ashcroft refused to extend Bush's program of warrantless wiretapping before, ahem, stepping down. And also because when high-ranking members of Team Bush met in the White House to approve torture as an American tactic - with Bush's full approval, no less - according to one top official, Ashcroft reportedly asked aloud after one meeting, "Why are we talking about this in the White House? History will not judge this kindly."
John Ashcroft, a man who once had a sheet placed over a statue's bare breast because he thought a marble nipple was indecent, a man who wrote and performed a song called "Let The Eagle Soar", a man who lost a Congressional race to somebody who died during the campaign, a Pentecostal man who believes he can speak in tongues when touched by the Lord...was the sane voice of reason in the Bush White House. He was the normal one. That is quite a legacy.