Ralph as leader
The boys on the island vote Ralph as leader for his natrual qualities and charisma. As the holder of the conch and the first one to call all of the boys together for a meeting, Ralph has a natural air of leadership that the boys are drawn to.
"But there was a stillness about Ralph as he sat that marked him out--there was his size, and attractive appearance; and most obscurely, yet most powerfully, there was the conch" (22).
Simon as visionary
Simon predicts that Ralph will be rescued:
"You'll get back all right. I think so, anyway."
Some of the strain had gone from Ralph's body. He glanced at the sea and then smiled bitterly at Simon" (120).
Simon has moments of foresight and insight throughout the novel. He is the first to suggest that perhaps the beast on the island is the boys themselves. Subject to "always throwing a faint" according to Jack, Simon's condition makes him vulnerable to dizzy spells, and perhaps even hallucinations such as when he spoke with the Lord of the Flies.
Roger as follower
"Roger, uncommunicative by nature, said nothing. He offered no opinion on the beast nor told Ralph why he had chosen to come on the mad expedition. He simply sat and rocked the trunk gently" (121).
Roger says very little throughout the novel. Portrayed as a school yard type bully who throws small stones at the littluns, Roger is an unimaginative follower. Under Jack's leadership, Roger's true instincts for violence and terror develop.
Ralph has better comprehending qualities of people than the other boys, which gives him leadership qualities. Ralph understands that all the boys have dignity and must be treated with respect and equality.
Simon is peaceful, serene, and positive, but he has not enough leadership qualities
Roger is not fit to be anything else, because he is so violent and he enjoys hurting things with animal instincts. He is too much of an executioner to actually have a job.