Very good point, but to answer the question, I would say that Simon would be the "good." He is the symbol of Christ, after all--the reference to feeding of the thousands and his symbolic death back that up.
Then to give an example of an "evil" would be Roger. I always thought he was worse. The way he played the role of the executioner so well has always given me the chills.
Well, the novel asks questions about what is good and evil in human beings. It is easy to say that Ralph is good and Jack is evil in the book, but both characters have flaws and both have redeeming qualities.
The question should not be what people are good and what people are evil. The question should be, are people generally or wholly good, or generally or wholly evil.
All characters, and all people, have flaws and bits of evil in them, but circumstance can either bring out the best or worst in them.
I would say that simon is the good one because of that fact that he is widely known to be the Christ figure in the novel. But i would say that Jack is the "bad" person just because he is the exact opposite if simon.
i agree with troutmiller