you can't really make a ranking
This is my view. Is a serial killer that kills five people a better person than a serial killer that kills ten? When it comes to the terrible atrocities like Hitler, Stalin, etc - they are all bad. The thing that is worst about them, is all the support they got along the way to get into those positions. It takes more than an individual to create an atrocity.
That's true, the support of a full society is what made them reach that point... Gladly not everyone was willing to obey and rebelled, like Albert Battel or Schindler