A favorite sport on the "left" is arguing over when the Soviet Union went bad. Council communists and such like believe things went sour from virtually the very beginning under Lenin when the Bolsheviks began to monopolize power. Trotskyists see it in the Stalin supremacy beginning in the mid to late 1920. Maoists believe it all went off the rails with the rise of Khrushchev after Stalin's death in the mid 1950s. "Moscow liners" think that the Soviet Union was still on the right track until is was derailed by Gorbachev.
For the council communists and anarcho-communists the problem was that power did not reside at the bottom with authority above delegated from below. This is certainly a view worthy of discussion under conditions of advanced capitalism. However, I do not think it has any application in a society of peasants and recently proletarianized peasants who are required to create a modern arms industry in extremely backward conditions in time to repel Nazi Germany.
Trotsky abhorred lack of inner-party democracy and the purging of the "old Bolsheviks" , the emergence of a bureaucracy loyal to Stalin and the betrayal of world revolution via "socialism in one country". This had lead to the Soviet Union becoming a "deformed workers state".
Indeed, the party and bureaucracy under Stalin had various pathologies. As for the differences between Stalin and his various opponents I don't know enough to say much. The exception would be the question of "socialism in one country". Trotsky's view was that such a thing was impossible and the policy was leading to betrayal of the revolution at the geopolitical level.
As far as I can see, the Bolsheviks were stuck with socialism in one country given that the chance of revolution in Europe was zero. What else could they do? This of course in no way contradicts the fact that the final victory of the communist revolution is a global affair.
Trotsky claimed that Stalin was putting the defense of the Soviet Union ahead of wider considerations of world revolution, and this showed up in Soviet diplomacy and in the policies they imposed on foreign communist parties. This is best addressed by looking at whether the various policies in dispute were correct or not and whether errors can be plausibly explained by an excessive concern to protect the Soviet Union. So far, I have looked at the French Popular Front and the German-Soviet Neutrality Pact in previous posts and given them my seal of approval. I will look at other cases in the future.
For Mao in China, Khrushchev represented the big and clear break with Marx, Lenin and revolution. His scatter-gun, wholesale and unprincipled attack on Stalin undermined the legitimacy of the whole Soviet "project" in the eyes of many individuals in the world communist movement. His claim that another decade of production growth would bring communism to the Soviet Union was the theory of productive forces at its most ludicrous. His claim that the Soviet Union was now a state of the whole rather than dictatorship of the proletariat turned Marxism to mush. We were assured that all that the stormy class struggle was a thing of the past. A quiet and peaceful parliamentary road to socialism lay ahead of us. Then there was his treatment of China. He sided with India in its border conflict with China and ended aid in abrupt and disruptive fashion. I think you get the idea.
Mao had his own criticisms of Stalin. The Georgian did not understand how a new bourgeoisie could emerge among those in power with a social base stemming from "bourgeois right" and the lingering old division of labor. Mao also believed Stalin confused the two types of contradictions – those between the people and the enemy, and those among the people – leading to excessive repression. Any sign of disagreement or disaffection made you a counter-revolutionary. Stalin was responsible for the "Wang Ming Line" that caused various problems for the Chinese communists in the pre-1949 period. I suspect Mao would have been a lot fuller in his criticisms if it were not for the fact that Khrushchev and friends were using the attack on Stalin to throw out the baby with the bath water.
Mao described the new situation in the Soviet Union under Khrushchev and then Brezhnev as the restoration of capitalism. Many have objected to this diagnosis, not just Moscow-liners. Unlike the case in China after Mao, with the establishment of what were obviously private businesses, large and small, on a massive scale, economic arrangements in the Soviet Union looked quite similar to how they were under Stalin.
On the other hand you could ask why would you consider a country socialist when the rulers are clearly happy running a class society and there are no signs of society being progressively transformed in a communist direction. Or you could say that when Mao was writing the capitalist restoration was in its first phase when the task was to turn "socialism" into something awful, worthy only of being replaced by capitalism.
I mustn't end without a nod to the Cliffites who believe that the Soviet Union was already capitalist in the 1930s and that this was evidenced by the fact that it traded with capitalist countries!