Rolling Stone's article The Worst President in History? doesn't pull any punches:

George W. Bush's presidency appears headed for colossal historical disgrace. ... Many historians are now wondering whether Bush, in fact, will be remembered as the very worst president in all of American history.

The article is quite thorough in comparing W's (in)actions and policies during his tenure to that of other widely criticized presidents, and although it stops short of labeling W The Worst President Ever. However, as detailed in the article (and through his previous and continued unforgivable incompetence), there is little that he has done that will commend him to much better than that.

But just because the article won't come out and say it, you can be sure that I won't miss the opportunity. Insert Comic Book Guy voice here: Worst. President. Ever.