That's so broad, it's hard to immediately think of any. The common 100 million number is refuted by the Black Book of Communism itself, though, considering some of the writers disavowwed it and Conquest includes literal Nazis killed during World War II as "victims of communism," before you get into falsifications.
Communism
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Hakim had a great video about that pretty recently, and he barely had to try to both refute the 100 mil number and come up with 200 mil deaths under capitalism.
"It's always been like that". And variations thereof. Engels shows it's untrue in an extremely easy to digest fashion in "Origins of private property". All one has to do is bother to open the book.
"There has never been a successful socialist/communist country"
It's like yeah, motherfucker, because the CIA has it destroyed to prove that "there has never been a successful socialist/communist country".
It's also wrong outright, there have been many successful revolutions.
I've never really heard a good anti-socialist argument.
Because there isn't.