I think the “most successful” post is the one that causes a positive change. Even if the change is by one person, and even if that one person is the one who started the thread in the 1st place.
Besides, there is no way of telling how many people read a post (or hear about it in the outside world) but do not post.
Some of the threads have been very powerful. I don’t think we could say which got the most mileage because the ripple effect is impossible to measure or track.