My favorite is James Madison, though most of the "experts" put Washington and Lincoln at the top.
Where does Obama rank? Most of those same "experts" put him somewhere in the second quartile, i.e. not the best but better than average. That strikes me as grade inflation on a massive scale.
Is he among the worst? Michael Medved says not, because Obama was re-elected. However, so was Ulysses S. Grant, and Grant is generally considered the worst or second-worst president ever.
There are also a few among the typical "worst" lists that don't deserve to be, like Calvin Coolidge. The "experts"--even the conservative ones, apparently--tend to value "greatness" as in scale of presidential ambition. That means a limited-government conservative like Coolidge will usually fare poorly.
My own feeling is that Obama's massive debt restricts him to the third quartile at best. There's the Osama bin Laden raid, but the Benghazi debacle is a good argument for the fourth quartile, along with his centralization of power and disregard for the Constitution. The economy is also a shambles, four years later.
Where would George W. Bush go? Prior to 2008, I'd say he had a shot at the second quartile, despite his big-government policies. His reaction to 9/11 was generally correct, and his decision to go for the Iraq surge took historic courage. But the bailouts--well, down to the third quartile you go. Though not to the fourth.