What made Marlon Brando a great actor?

I am not trying to throw the hate, but why do some people consider Marlon Brando one of the greatest actors ever? What made him a truly great actor? I’ve seen a couple of his films and he was just mediocre actor to me. He was solid in The Godfather, but Oscar-worthy? I really don’t think so. After I heard about that he raped Maria Scheider for real, I completely lost respect for him and he didn’t even apologize to her. That’s why I can’t see his face in his films because of that. He was just a pig and never impressed me anyway.