I’d say it depends on the particular regime. Hitler, Stalin, Mao, Pol Pot, and Kim were the worst. Conventional fascists and parafascists (Mussolini, Franco, Salazar, Peron), Titoism, and revisionist communists less so. Though all of them still sucked. And let’s not forget the centuries-long myriad of crimes carried out by imperialists, colonialists, and capitalists.