None of these people are even close to being the worst presidents ever. Especially Harding.
I admit that Harding and Carter might be stretches, but surely Buchanan and Pierce are worse than Obama.
Funded rebels
Kept war in the middle east intentionally
NDAA
Extensions to Patriot Act
Obamacare
all make him much worse than any of the people you listed. Including Pierce and Buchanan who weren't that bad.
So in other words one bungled health care reform and four things that literally every President would have done and has done since the US was funded; and that makes you worse than a massively corrupt guy who apologised for slavery by screwing around with Dredd Scott case, almost broke up the union (e.g. Marill Tariff); and effed up on numerous occasions (Star of the West, the Utah War, etc.)?