Better question: Least horrible. Regarding that list, of course.
Make your own thread instead of hijacking mine:-)
Most horrible is interesting because of the different criteria you can apply.
Hitler was responsible for the highest number of deaths if you give him sole responsibility for WW2. Mao killed the highest number of his own people (if you include starvation), Pol Pot killed the highest percentage of his own population and ruined his country's economy and social structure. If you combine the two criteria of killing the highest share of your own population and the highest number of your own population Stalin wins. Stalin also probably had the largest longtime negative impact and imprisoned and forcibly relocated most people. But Hitler is the conventional choice as the personification of evil. I would say Stalin if you combine all factors and Pol Pot if you focus on the amount of evil a dictator caused relative to the resources he had at his disposal.