Would you have dared say that to his face? Therein lies your answer
Not really a competition I'd like to pick a "winner" in to be honest. In terms of sheer overall numbers of deaded Stalin probably has it. In terms of proportion of population Pol Pot probably does. In terms of sheer insanity that fella who banned ballet dancing (amongst other things) who is the dictator of one of the lesser-known 'stans (sorry, gave up checking which one it is/was (think still is?) but is one of 'em all the same). Hitler is probably most notorious but is fairly middling in terms of "achievement" by true nutso dictator standards. And this is all leaving aside some of those African dictatorship nutzoids. And historic ones.
I'm not sure I go along with "evil" as a concept but if I did they're all much of a muchness bar the outright fruitloops (mental illness is not "evil" no matter how horrificly enacted).