Hmm... That's a somewhat complex question and I would probably go this way if I had to:
Marx is obviously first - despite a few trends that he missed due to the world of his day, and his inattention to practical affairs.
Lenin is #2 because he fleshed out Marx's analysis of the state, and how it functions, as well as how Marxian groups should behave in response.
Stalin is easily above Trotsky - as most of what Trotsky *actually* wanted for the USSR was far more negative than most of what Stalin actually did. Trotsky benefits somewhat by having not actually done all that much, once state power was achieved, besides being exiled - and then complaining a lot about it.
Then Mao - if nothing else than for his penchant for not doing what Stalin wanted him to do - and carving a path for independent action (from Moscow). Also his writing on how to execute a guerilla war is still a standard textbook throughout the world (including the US) so that's gotta count for something.
Lastly Trotsky - as I mentioned previously, most of what he actually wanted for the USSR was demonstrably worse than what Stalin actually did - there's a reason Stalin won out in the end despite being a non-charismatic and often disagreeable fellow. In spite of Trotsky's contributions, militarily, during the revolution.