"Why is fostering good conscious feelings and prevent bad conscious feelings necessarily correct? It is intuitive for humans to say we should maximize conscious experience, and that falls under the success theory that Peter talks about, but why is this necessarily the one true moral system?"

If we agree that good and bad feelings are good and bad, that only conscious experiences produce direct ethical value, which lies in its good or bad quality, then theories that contradict this should not be correct, or they would need to justify their points, but it seems that they have trouble in that area.

"But valuable to who? If there were a person who valued others being in pain, why would this person's views matter less?"

:) That's a beauty of personal identities not existing. It doesn't matter who it is. In the case of valuing others being in pain, would it be generating pleasure from it? In that case, lots of things have to be considered, among which: the net balance of good and bad feelings caused from the actions; the societal effects of legalizing or not certain actions...