In my opinion the catholic pope is the most extreme dictator in the history of mankind.
Here are some of my reasons:
1) He thinks he is INFALLIBLE.
2) He issues dictates telling people WHAT THEY MUST BELIEVE and HOW THEY MUST LIVE.
3) According to catholic "morality" if anyone rejects even one of the pope's dictates in matters of faith and morals, they will be not merely executed, they will be TORTURED IN FIRE FOR ETERNITY.
Thus,in my opinion Stalin, Hitler and Mao were not even in the same league as the pope as extreme dictators.