Nazis: ~6 million
Soviet Union: ~20 million
China Cultural Revolution: ~30 million
Though the Soviet Union and China ones were more of a result of incredibly bad economic policies. The Soviets still killed ~1-2 million in Gulags directly.
Let’s also not forget the Japanese genocides against China. Nanjing, Unit 731…
There also is, of course, more recent ones that are little known. Cambodia took over a million and Rwanda close to that.
0: http://web.archive.org/web/20160709210523/https://www.svobod...
In contrast, imperialism and colonialism fucked people over all over the world. And some of the later impact is masked, because they enabled locals to do the dirty job for them - to the tunes of tens of millions affected.
The Nazis thought they were fucking their own people. Every oppressive power uses this excuse.
After many revisions the total death toll at each of the concentration camps where the Holocaust occurred (which just so happened to only occur on ex-Soviet territory) adds up to a tiny fraction of 6 million, but the overall "6 million" Soviet propaganda term was never revised.