Well I guess that depends on how you measure
If it is by gross number of its own citizens killed most historians would give the title to Communist China.
If you go by percentage of the country’s population killed most historians would give the title to Kampuchea (Cambodia).
The Soviets were no slouches when it came to murdering their own people but in either of the above measures they come in second but they leave Hitler’s Germany in the dust.
I guess the reason history picks on Hitler to the near exclusion of Moa and Stalin best said by Churchill “History is written by the victors.” Not to mention that most of the press and college campuses in the West are populated by communist sympathizers.
All of those regimes have something in common - Marxist hellholes. You forgot the hundreds of thousands who died during North Vietnam’s “agricultural reforms” which were inspired by PRC agricultural policies in the 1950s.