Nixon, morally corrupt, a true danger to the Republic in my view. He is redeemed in the last case by some of Kissinger's diplomacy vis a vis China.
Harding was absolutely worthless. The level of blatant corruption inside his administration may ultimately be topped by G.W.B's--but this is one area that is still open to debate in my view.
The history on G.W.B is not written yet. He may very well prove to be the worst, but a lot hinges on how Iraq goes. Iraq has the potential to be much worse than Vietnam, but we are not there yet. G.W.B's abuses of office are not quite at the Nixon level yet; although this could change as well too.