I think that, except for Guiliani, any of the Republicans running -- Romney, McCain, Huckabee, whoever -- would be as bad as Bush has been. (And bear in mind here that Bush has been the worst president in U.S. history.) With different emphases probably -- a smidge better here, a bunch worse there. But overall, equally bad. (Why is a longer topic than I have time for in my current blog slowdown, but I think it has to do with structural forces at play in American politics today, especially on the right, rather than anything about these men as such.)
But Guiliani has the unique distinction that he would be distinctly and clearly far worse than Bush has been.
The unparalleled disaster that a Guiliani administration would be shouldn't blind us to how terribly, terribly bad the others would be; conversely, the fact that every single one of them would tie the worst president in history shouldn't blind us to the fact that one in particular would be even worse.