Well, let me make the caveat first that there's no absolute right answers to this one, and it depends on how you read the book...
Piggy. The easiest to symbolically analyse. Common-sense, intelligence, democracy and "clear-sightedness" (his glasses, of course, providing fire - and rescue). The chief proponent of the conch.
Jack. "I'm chapter chorister and head boy", he says, right at the start, a little clue from Golding that he was top dog at school, and he'll want to be top dog here. Jack effectively performs a back-handed, thuggish political usurpation of Ralph, replacing the civility and democracy of the conch-law with fear, violence and a tyranny in which he is the all-powerful "chief". I always find it difficult to forget that "Lord of the Flies" was written only shortly after Hitler's rise.
Ralph. Is the everyman figure, who goes on the traditional "rites of passage" journey from innocence and worldly optimism ("My daddy's a pilot..." and he's going to save us!) to realising the "darkness of man's heart". He doesn't, I would argue, stand for anything other than you and I - the "normal", everyday guy.
Simon. A strange, weak, Christ-like figure who knows the right answers but can't tell them to people. Very intelligent, sensitive and perceptive, but not a natural leader - the artist in society? A real paradox.
the religious prophet or seer who is sensitive and inarticulate yet who, of all the boys, perhaps sees reality most clearly.