It all depends how you define “worst”.
In terms of sheer failure, it’s Carter (and that includes Obama SO FAR) or maybe James Buchanan.
In terms of damage done to the country, it’s unquestionably FDR (and that includes Obama).
And yet I want to say Obama is worse than any of them.
The glaring commonalities all of the POTUS candidates considered as “the worst” are all RATS. Possibly more damning than that is they span different centuries, proving s*** is s***, no matter when.
I might rank LBJ ahead of both of them in terms of damage done to the country, even though LBJ had half the time as FDR to do it (6 years vs. 12 years). 1963-1968 was a horrible disaster for America and we've never recovered from it.