@coldjoint,
In the Netherlands, there was an election of the greatest dutchman (or woman) of all time in 2004. As fate would have it, less than two years prior, a prominent right wing politician, Pim Fortuyn, had been gunned down when he left a studio where he had participated in a talk show. At that time, he had certainly stirred up a great deal of controversy, but in terms of what he had done for the Netherlands, that was next to nothing, since he had never even lived for the first elections he would participate in. (his political party won very big that year though).
So, back to the show. When the votes were counted at the end of the program, for a while, it was believed he was effectively number one. In the end, apparently, due to a miscount, he was placed at number two, and William of Orange was made number one.
The reason I'm pointing this out is because of the simple fact that people tend to have the strongest feelings for the most recent past. Which in effect means that this poll is polluted. It's the most recent president on the list who gets the most votes, because he is the one people best remember. A close number two is Bush, whom was president before Obama of course. I reckon the 13% for Nixon were due to 'Watergate'.