Hard to think of anything he's done particularly well.
I'd have to say tax cuts or Afghanistan, However he managed to fuck up both of them. His tax cuts were poorly structured and the Afghanistan campaign was cut short and essentially unfinished, and as a result the nation isn't nearly as stable as it could have been.
Both were good ideas poorly executed.
Worst?
I'm tempted to say getting rid of Treasury secretary O'neil for being an honest man, and replacing him with a criminal who doesn't mind "deluding the people" and "pandering to the lowest common denominator"
The various infringements on liberty in the pursuit of security in total.
Spending all the political capital, global goodwill, and momentum the conservative movement had post 9/11 on a poorly executed invasion of Iraq, trading a secular dictatorship for SCIRI, and abandoning our bases in SA to do it.
Borders.
Apparently completely forgetting that Russia and China exist and that they should be a consideration when making policy.
But the number one worst thing president Bush has done is the way he's handled Iran, and the way he's decided to deal with them in future.
He put Amadinajad in power, and we'll suffer to take him out.