Well in terms of numbers, Mao undoubtedly is number 1, with probably about 50 million deaths. However, it is true that he had more people to kill than the others.
As for the "worst dictator category", it would be Hitler for me. I study in history and I have had courses on both Stalinist Russia and Nazi Germany and as much as Stalin was evil, Hitler was just worst.
Stalin killed lots of people because he wanted to assure his own legacy and safety. He was a paranoid who killed everyone that was in his way to guaranty his own welfare. Hitler, on the other hand, killed millions of people for the alleged science of eugenics. He killed many for their skin colors, religions, mental abilities, etc.
Stalinist Russia, as awful as it was, was not worst than Nazi Germany.