This is of course a very subjective question and your answer to it is going to depend on a number of other questions concerning the story and how you interpret it. Is it just a dream, or does it have some basis in reality? This is a key question that will definitely influence your answer to the question you have asked.
From my perspective, I actually think that strongest character is Faith and the weakest is Goodman Brown. If you consider that Goodman Brown's experiences were a dream, it is Faith who shows herself to be faithful and true and the strongest of all the characters. She is able to sense that something evil is going to happen that night, and entreats her husband to stay, but also accepts his decision to go. In spite of how he is transformed by his experience, we are told that Faith remained, aptly enough, faithful to him and they lived all their lives together.
Goodman Brown in my opinion shows himself to be the weakest character because of the ease in which his experience transforms him. His revelation of the evil within us all sucks all joy and happiness out of his life. Note how he is described at the end:
A stern, a sad, a darkly meditative, a distrustful, if not a desperate man did he become from the night of that fearful dream.
So dark and gloomy does he become that we are told that on his death no "hopeful verse" was carved onto his gravestone, because after his experience his life was characterised by gloom and nothing else. To have been affected so profoundly and then to make the lives of others a misery shows a weakness and perhaps a childlike innocence.