If you rank Hitler as first, then you are not only considering their ideas and personality, but also their power/influence and thereby their impact
I would argue there have been worse persons than your top 3 But they lacked power, so they have been doing their utterly despicable things at a very small scale.
Please define worst people - as in evil? Then you could at a Jeffrey Dahmer or serial killers such as Columbine killers or Newhook (Adam Lanza).
There are so many in history - look at someone like Ghenghis Khan, Pol Pot, Kim Jong Il (I know these guys are from the East) or some of the African dictators like Robert Mugabe.
History is filled with powerful and charismatic people that are twisted.
