I would have to say in my opinion Stalin was the worst, he had more people killed and he had more decades to be in operation.
Now Hitler, the mad Austrian. He was evil and wicked too, as he killed innocent people for being a different race, religion, way of life etc,
However I have to say, he only had 13 years, oh my goodness, imagine if he had more time.
So Stalin wins cos he had more time, but that's only cos Hitler didnt, so who knows what would have happened, thankfully he didnt finish his "agenda".