Stalin, and mao tse tung.
Stalin and Mao Tse-Tung did not kill millions, these numbers were exaggerated/made-up by Deng Xiaoping and Nikita Khrushchev.
The destruction Hitler did was massive. He not only killed millions of people in his death camps, but his armies murdered millions more, destroyed Europe and devastated the people there.
Hitler also ruined entire generations of Germans, and left a horrible past that Germany has to accept as it's history.