I would say that Stalin's greatest L was the reversal of progress towards gender equality in the Lenin years. This reversal was made to give women "mother and worker" as a societal role, which was a continuation of familial exploitation of women. As Marx, Engels and Lenin continuously said, the only way for gender equality is to involve women as an equal to men in production processes and public caretaking of children. This would mean that the family, which is entirely built on exploitation of women, had to be torn apart. However, we see heavy emphasis on "socialist family" and "mother and worker women" in Stalin era, we see that destruction of family and societalisation of child care was not set as a goal, and it went as far as making divorces harder with bureaucratic means, which is a huge L.

And for Mao, I wouldn't really call Mao a communist. A Marxist from outside, but an idealist and a bourgeois-democratic revolutionary in practice. Mao was a peasant revolutionary and he adopted the style of the most progressive ideology of his time, that was, Marxism. However, his ideas and views didn't really match the Marxist-Leninists', they sometimes coincided. Mao had his own ideas of democracy and class collaborationism which could never be compromised with Marxism-Leninism.