Who was responsible for more state-sponsored murder? Adolf Hitler, Joseph Stalin, or Mao Zedong? Any stats to support your assertion?

Adolf Hitler seemed to focus mostly on Jews, Gypsies, mental patients, communists, and anyone else that gave him trouble. Stalin killed anyone that seemed to be a rival for power. He might have started a second holocaust against the Jews (cf. The Doctors Plot) but he died before it could reach fruition. He certainly had something to do with the enforced starvation of kulaks and the forced collectivation of agriculture, which was not a gentle process.

Mao did essentially the same thing in China. Maybe because he had more people to work with that was the reason he was able to kill more of his own people. The cultural revolution certainly did something memorable to the Chinese people.

Mao.

Look them up yourself, there's more than ample information available.

Stalin came in second, Hitler was a distant third.

Nothing kills like collectivization for the "common good".

Mao

Creating a league of the world's worst dictators is futile and juvenile.

Each deserves to be looked at separately and in context.

Hitler

Mostly because you chose to use the word "murder" which would of course not include deaths from stupidity, famine, or incompetence.