1) FDR No dispute there.
2) Lincoln - I like Lincoln, but I've had a tendency to rate him lower ever since hearing a pretty convincing thought experiment once. Imagine that, instead of the Civil War happening, Lincoln had found a compromise that would contain slavery and then allow it to be phased out gradually over the course of a decade. Would Lincoln be rated higher or lower? He should be rated much higher, for having accomplished the same thing with vastly less bloodshed and destruction. But we tend to give extra credit for those who presided over parts of history that make entertaining stories, and civil war is highly entertaining. So, I mark Lincoln down just a bit for the fact his greatness is almost entirely linked to his handling of a war that might theoretically have been avoidable. FDR, too, is remembered as a wartime president, but much of his greatness had to do with his peacetime leadership, in the New Deal.
3) Theodore Roosevelt - I'd rank him higher, but something about his egotism makes it hard for me to do that.
3) Adams - I rank him quite high, too.
3) Truman -- With him, I give him the lion's share of the credit for the overall peace of the second half of the 20th century and the ultimate triumph of democracy over Soviet communism. His policy of containing communism without provoking it (for example, defending South Korea without provoking China into a full-scale war), ending up being the "secret sauce" of American ascendence. Combine that with the Marshall Plan (which set the global economy up for spectacular growth), the UN (which helped to defuse many humanitarian and global politcial crises), and strong social policies (such as the integration of the armed forces), and it's pretty impressive.
5) Carter - I think he may be the most underrated President ever, but that's not enough to get him into my top 10. His time was just too rough, economically, and he set the nation up for the rise of Reaganism, which was disastrous. I also think his big accomplishment in foreign policy (Camp David) is somewhat offset by helping to set us up for future problems with Iran and Afghanistan.
6) LBJ - I could rank him really high or really low, depending on whether I was thinking about social policy or foreign policy.
7) Washington - I think a lot of his decisions as president were poor, but he was really important in setting up so many traditions, and in establishing the notion of limited presidential powers, that I give him a free pass on most of his decisions.
8) Clinton - He's sort of the flip side of Lincoln. He gets rated lower than he otherwise would because his era doesn't make for that much entertaining history. There was no great war, nor any huge crisis for America to overcome. Yet good leadership might make it possible to deal with potential problems before they become crises. Maybe in some alternate universe Clinton lost in 1992 and we got another Great Depression under Bush, or neocons provoked the Russians to the point that Zhironovsky would have succeeded in taking power and we would have gotten a war. It seems unfair to penalize Clinton just because eight years of peace and prosperity don't make as good of reading as battles and disasters.
9) Adams - I score him high because he helped keep America out of war, at a time when it was vulnerable enough that a war may have ended the whole experiment when it had hardly begun.
10) Jefferson -- I give him credit, but only grudgingly, since I see his vision of America as regressive. HIs agrarian utopia just wasn't where the country's future lay.