How does one measure that? They were all murderous beasts who couldn't kill enough people. Joe said a single death is a tragedy; a million deaths is a statistic. But since you asked the question, I'd say that the self-hating person who gave life to the idea of communism should come out as the worst.
I recently read that Marx was a Jew who hated Jews. He hated almost everyone, above all, he he had an intense hatred for Almighty God as well. His own writings show that if he could, he would destroy all of humanity and follow them laughing into Hell.
Looking into the roots of communism, I wonder how in the hell any person of Jewish heritage could even entertain the notion of becoming a communist, let alone actually embracing that ideology.
And then there are the so-called Christian communists. What an oxymoron that is.
Karl Marx was a prime example of who NOT to emulate. He was a parasite who attached himself to any host who would support him. He did nothing for himself, except try to separate his friends from their money.