...lolwut?
There have been some very corrupt US Presidents. Harding comes to mind. However, Jackson and Wilson outpace the rest of that list pretty damn heavily and even those examples could be considered morally gray, not evil.
Saying "Nixon is most evil!!!oneoneone" shows a very narrow-view of history. There are few "evil" leaders in the world. Does Mao Zedong or Stalin really compare to the Black Pajama Party of Cambodia or Hitler? No, they don't. Not in the slightest.
Mao Zedong has killed many more people than Hitler. So actully he should be considered more "evil."