I don't see how people are saying Bush43 is worse than Harding and Hoover. Those two go on their own plateau--it's that that the people lving through Bush tend to exaggerate his ineptitude as they have experience it themselves and not just read some articles on the internet.
Many of these people have no historical perspective. I think more recent presidents should be left out of these surveys because there's no way to judge a president still in office, or one who has recently left.
Those who say Bush 43 is the worst president are only revealing their own biases and lack of historical perspective. The jury will be out on his presidency for some time after he leaves office. Many of these people probably don't realize that many presidents now considered great were thought of in the same way when they were in office. Harry Truman is the best example I can think of. Except for his first year or so in office, Truman was highly controversial and pretty unpopular for most of his nearly eight years in office.