Ok first of all Stalin wasn't a Communist. If your ideology is based on killing the bourgeoisie rather than actually helping people, you're a sorry excuse for a Communist. Stalinism was a way of achieving industrialization much the same way as cannibalism achieves a high-protein diet.
Same goes for Mao and Pol Pot. Ok I agree that some of their moves (such as gender-equality) were liberating. However, looking at their other moves such as Russification under Stalin and dominance of Pinyin and Mandarin Chinese on other ethnic groups such as the Uyghurs and the Tibetans and going as far as culturally genociding them, THAT is complete bullshit.
I would have to agree that yes, Stalin's legacy did lead to nuclear weapons, scientific development, and gave rise to a much-needed balance of power during the Cold War or else the US may have asserted their dominance globally, however, that is not an excuse for killing 10million+ people.
Mao was a hypocrite: he criticized Khrushchev for having a deténte with the West and said that "peaceful coexistence" was "revisionist bullshit". This guy's response to the Cuban Missile Crisis was "Why didn't you 2 go nuke each other." He then goes onto developping relations with the US and Nixon visits him in 1972. His ideology, especially that of cultural revolution and the GLF was an utter failure which caused him to step down (GLF, that is).
Gorbi was just too naïve. He did too much, too late. Ok I understand you wish to reshape the economy like Deng, which is a good idea that I would support, however, you want to democratize too? The criticism developped from Perestroika effectively got amplified by Glasnost. If he had been like Deng and been authoritarian, the USSR could have been saved.