What is a good general rule of thumb for knowing when to use who/whom?

Whom is the object form of who. Just like him and he. The object is when something is being done to it and the subject is when it is doing something. for example, he throws the ball. He is literally throwing the ball. But if the sentence were the ball is thrown at him. He is not throwing the ball and you the ball is being thrown. the same thing applies to who and whom. if I say who is the greatest of the all? I am asking who actually is so that is a subject so we use who. But if the sentence were wait you like whom? That is the object form and we whom because we're saying you like a person, they are being liked.