The difference, in my opinion, between Hitler and Stalin/Mao is that Hitler led through arguably the greatest cult of personality the world has ever seen. Stalin and Mao ruled out of totalitarianism-inspired fear. Stalin and Mao were not great orators, while Hitler was - without question - the greatest orator of the century, and debatably all-time. Churchill and MLK are a distant 2nd/3rd. In Reich Germany, there were free elections that Hilter won by absurd numbers (yes, you could argue that it was somewhat corrupt and influenced by the Stasi, but that aside he was beloved). Hitler's rule in the 30s was the prime reason behind a world war, and people flocked to him like he was a human banner.
Death numbers for Stalin and Mao aren't even closer to Hitler, that's absolutely true. However, the reason they're so high is not the same.