Jack and Ralph both are tallest guys.They both have the ability to become leader.They both have strong will. The decision they take, can put into effect without any hesitation or fear.
Jack is full of violence.when he issue order he punish, use harsh words, do physical violence.On the other hand Ralph is a nice guy, believes on democracy. When he issues order, he respect the majority's opinion. His decisions are opinionated. He put the problem in front of meeting and gets feedback to act upon.
Ralph is a wise and hard worker while Jack as a tribe leader depends upon other boys. Ralph believes that Jack is a honest good guy while Jack believes that after piggy's death he would create trouble for his leadership. His behavior shows greediness and selfishness. His motivation is just punishing,and killing people. Moreover, Ralph is a responsible person. He showed his responsibility by thinking about collection of water, building shelters and allocation of toilet areas.
Jack is selfish, greedy, lazy and irresponsible person so he is an ineffective leader while Ralph is a good, honest, responsible and democratic person so he is an effective leader.