I think that John Proctor actually changes the most. From start to finish, he is the only character we see throughout the entire drama. He is in every act, in nearly every scene, and we are able to gain the most from seeing him down the line experience a gradual change that crescendos at the end. At the start of the play, he does not seem certain of what he wants. In his first interaction with Abigail, the stage directions indicates that he "smirks" at her and almost talks to her as a "children will be children" routine. While he does oppose her advances, he does not seem committed to his course of action. In the second act, his adding spice to the broth is reflective of the fundamental tension he feels in his marriage, snapping at Elizabeth when she prods him to divulge what he knows to Cheever and the court, and demonstrating a noticeable rift between his guilt, her hurt, and the bind of marriage that holds and eventually cradles them both. In the courtroom scene, we start to see the first time that passion and intensity towards a noble cause enter Proctor's calculations as he admits to the affair with Abigail in public, yells across the courtroom for his wife to tell the truth, and curses God in front of all to prove that what is done in the name of religion is not in fact religious. The closing scene where he "confesses" is the last time we see a figment of his old sense of self, for when Danforth tries to compel him to sign it, John intensifies his stance and rails against everything that we, the viewer, have been subjected to throughout the drama. He speaks what we, as the audience and viewer, want him to speak and wish/ hope we could speak if confronted with the same reality:
Because it is my name! Because I cannot have another in my life! Because I lie and sign myself to lies! Because I am not worth the dust on the feet of them that hang! How may I live without my name? I have given you my soul, leave me my name!
There is nothing quite like this language displayed at the start of the drama. Proctor never speaks any words of both this emotional magnitude as well as the forceful nature of asserting both his identity and his place in the world. It is a moment where Proctor becomes more than a man, and almost more than a human being. In his notes as he was preparing to write The Crucible, Arthur Miller jotted down: "It has to be about Proctor." Indeed, Proctor is the central point of the play. It is here where change begins and is the most evident. It is in Proctor where we understand the fundamental difference between who we are and who we need to be. It is in Proctor that there is clear evidence that there can be an aspiration of how the world should be in the face of how the world is. It is in Proctor where we see a "change we can believe in," to borrow a politicized version of the concept of change.
We might argue that Elizabeth is the character that changes her course to the greatest extent.
Initially, she is defensive, petty, and cold in he relations to John Proctor. As the play goes on, Elizabeth softens and demonstrates a growing understanding of her husband, as well as forgiveness. Further along, she goes so far as to lie to protect her husband. Later, she has come full circle as she tells John that it is okay to refuse to sign the confession. She places her respect for her husband's integrity above her desire to keep him alive.
I would second Reverend Hale. He arrives clearly convinced that Satan is in Salem, and sincerely deals with the people there to try and root him out. He is perhaps the first of the Court authority to come to terms with the fact that there is no Devil in the village, and that the death warrants he has signed are unjust ones. This weighs heavily on his conscience and he attempts to save Proctor's life.
I would suggest considering Reverend Hale.
He arrives in Salem feeling like the work he does is blessed by God and logical and reasoned. If you look at his first entrance, he responds to someones comments about the weight of his books by saying that they are weighted with authority (namely the authority of God). As the accusations go on, he feels that those who are accused have nothing to fear if they are truly innocent. By the end of the play, he is begging John Proctor to confess to the charges of witchcraft to save his life. He tells John to give the judge the lie of his confession because it may be less of a sin to lie than to die to save his pride.
I think its both John Proctor and Rev. Hale.