Among all the dictators that ever existed, which one would you deem to be the worst and why?

People loathe the pissing contest between Hitler and Stalin that always arises with these questions, but it’s curiously absent from this thread. So:

Stalin.

Because with Hitler, at least you knew who the enemies were throughout his reign. With Stalin, the enemies changed depending on what side of bed he got up on. If both are evil, I’ll fear evil + unpredictable more than evil + predictable.