who has the most power

I am not sure what your power pyramid is, but ultimately the "girls" who cry out against the witches have the most power because their testimony can convict a "witch" regardless of what others say or do. Of the girls, Abigail has the most power because of her extensive peer pressure on the girls and the threats she levels at them if they do not go along with her. The Judges have great power, but they are somewhat arrogant about that power until Abby considers herself above them and begins to accuse other people high up. Ultimately, the people who have the least power are those who proclaim their innocence and who are, in fact, guilty of nothing more than being "different" or being hated by someone.