In my lifetime, I put Obama at 2nd worst . . .
LBJ was the worst, thanks to >58,000 dead Americans in Vietnam, a war in which his decisions actively aided the enemy. And his "Great Society" is the #1 source of today's national debt, and all the problems that go along with it.
BHO . . . what can I say? He was the 1st POTUS in which I could never recognize any shards of patriotism - his entire philosophy seems to revolve around diminishing the USA. 2nd worst POTUS overall, since he doesn't have as much American blood on his hands as LBJ.
During his term, I viewed Carter as well meaning, but naive and inept. For a time after leaving office he did some good (Habitat for Humanity) but later became anti-semitic and, well, wacky. So he still gets a solid 3rd worst.