If George W. Bush is one of the five worst presidents in U.S. history and the worst president in modern history, then his twin brother, Obama, has to be virtually tied for worst. Since the U.S. has evolved into this century's counterpart of Nazi Germany, it is perfectly appropriate to compare Bush and Obama to Hitler.
Though I also tend to regard Hitler as the absolute worst, I'm not entirely certain. How many people WOULD Bush have murdered if his hands weren't tied by a diverse, democratic nation and a family of nations that has matured somewhat since World War II? How many people WILL Obama murder? The SOB has already escalated the war in Afghanistan and is itching for war with Iran, which could easily turn nuclear. He's also beefing up the U.S. military presence in Latin America.
And let's not forget crimes against the environment. Everywhere the U.S. military goes, it leaves a trail of destruction - nuclear fallout, Agent Orange, depleted uranium, etc.
Though far from perfect, I wouldn't put FDR in the same category. In fact, Billary Clinton would be a better fit, as would Bill Gates. Though neither are commonly seen as murderers or warmongers, they're both utterly corrupt and are extraordinarily talented at screwing millions of people.