There are only a few characters that have power. First, the owner of the ranch has power. This would be Curley's father. Not much is mentioned of him, but when he is mentioned, it is clear that all men on the ranch look to him as the final word.
Because Curley is his son, Curley also has power. This can be seen, because the men tolerate him, even though he is the character whom most people would love to hate.
Another character with some power is Slim. He is the leader of the workers and the men do look up to him.
As for people without power, I would say the migrant workers have no power. They stay for a while and move on. They come with nothing and leave with nothing. They are living to survive. Lennie and George fit this category.
The person with the least power is probably Crooks. This is not only because he is a worker, but also because he is a black man. He, therefore, lives isolated from the other men. This spatial location says that he is really nothing.