John Proctor is the tragic hero. He is a hero because in the end he stands true to his beliefs and his philosophies, but in doing so he is condemned to hang.
As far as his tragic mistake, which time? He has an affair with the servant girl (Abigail Williams) which causes a strain on his relationship with his wife. He waits too long to attempt to turn things around in town when the girls have begun accusing so many of witchcraft. He confesses to seeing the Devil in order to save his life and live with his wife and children, but then goes back on it since it is a lie.
As a long-standing and respected member of the town, John Proctor has much influence on the other citizens. He and Rebecca Nurse hang as an example to the town--they hang on principle.
I would say John Proctor because in the end he has to make the decision to give up his friends and his name or live. His mistake was having an affair with Abigail Williams because this influences his behavior for the rest of the play. He and Elizabeth are estranged, and John feels guilt for doing what he did. He spends the whole play feeling guilty and it's not until the end that "He have his goodness", according to Elizabeth. She won't talk him out of hanging because John has made the right decision to deny he's a witch.