Defining "greatest" as most influential, here is a better ordering.
- George Washington - You know, Father of Our Country. Managed to united 13 feuding states into a single union. Put a lesser man in the office first, say John Adams, and the United States would have never survived.
- Franklin Roosevelt - Kept the country from unraveling during the Great Depression. Many of his programs continue to survive despite Republicans best efforts and those that have been lost (the Glass–Steagall Act) have been sorely missed.
- Abe Lincoln - Held the nation together during its Civil War. Another man (like Buchanan) would have given up the fight or not fought at all and the United States would have been torn asunder. Then there was that abolishing slavery thing.
- Thomas Jefferson - Doubled the size of the nation. Was a vital counter-weight to the anti-democratic Federalists.
- Ronald Reagan - Bankrupt the Soviet Union (although bankrupted the US in the bargain). Mostly he's here because a third of the nation has deified him and that has to count for something.
Who Doesn't Make the Cut: Clinton (Come on, what did he do?), Kennedy and Obama (Not enough time in office).
Hat tip to Rising Hegemon