In my opinion, the major conflict in this novel is the conflict between people's "good" and "civilized" instincts on the one hand and their "bad" and "uncivilized" instincts on the other.
I would say that Ralph and Jack symbolize this conflict. Ralph symbolizes the good and Jack symbolizes the bad.
The conch, of course, is a symbol of civilization as well. That is because it is used as a way to try to civilize the boys -- it is used to call them to meetings and it is used to make sure that they do not talk out of turn.
I'd say that Piggy and his glasses symbolize intellectuals and science and how they are used by society.
Simon represents morality and is seen by some as a Jesus figure.