I find it interesting that so many people are putting Hitler on the list.
The only thing he will be remembered for, is being a crazy guy who killed a lot of people. If that's your measuring bar, there are a lot better people for that title.
In Genocide, Hitler killed 12 million people. Stalin killed 23 million, and Mao Zedong killed between 49 and 78 million people.
So I would replace Hitler with Mao on everyones list.