Kristof thinks it's a tie between W and James Buchanan for the worst. In fact, it's probably early to evaluate W definitively, but I don't think he's the worst of the worst. My candidate for the worst is Woodrow Wilson, whose intervention in WWI was a key link in the chain to the disasters of the murderous Twentieth Century.
The best, besides Washington? Probably Calvin Coolidge. The historians traditionally give high marks to wartime Presidents, at least in victorious wars. The ones who avoided war, avoided dangerous innovations, and presided over domestic tranquility are underrated. This is not the time to revive the whole Lincoln controversy--great man or tyrant?
Kristof's other theme is his vision of internationalism. Aside from a rather naïve reverence for international organizations, including apparently the thieving UN, Kristof, like the neocons, seems to accept the notion that this country has some kind of high moral calling in world affairs, and should meddle abroad. Kristof simply wants to be nicer to other countries and more respectful of international institutions. Why not sharply reduce foreign entanglements of all kinds? It's not a question of unilateralism vs. multilateralism, but of hubristic activism versus a cautious modesty.
Stay out of other people's business, and keep our powder dry.