You have to be careful in how you define leadership, though. Successful leadership could simply mean the ability to get people to follow you. In that manner, Jack would be an excellent leader also (of course, so would Hitler). If your question is simply asking which boy would make better, more sound decisions in a leadership role, then your answer is certainly Ralph without question. But in terms of building a following, Jack was much more effective.
Ralph is more kind and level-headed. He sees the big picture and assigns jobs so that the picture can be drawn. He is not without a sense of fun, but duty comes first. My vote is with Ralph.
Jack rules by fear. He taunts and punishes. He doesn't care for order or rules, only fun...HIS FUN. He is selfish and cruel. He is any number of the tyrants who have ruled in the world and have been made to stand down by other more humane leaders.
Ralph is the moral, orderly leader who is interested in productive leadership, and hopes for rescue.
"The fair-haired, tall, handsome Ralph is an obvious choice to lead the band of children stranded on the island. He has a "directness" in his manner that the narrator calls a sign of "genuine leadership."
Jack is a representative of the savage instincts that emerge from the boys. He is physically stronger, more intimidating. He lusts for power and uses the boys to feed his desire for power. Ultimately, Ralph becomes subordinate to Jack, almost falling into his savagery, but resists. Jack is also a natural leader, he proposes that the boys call him chief.
"It's true that Jack has the advantage of being tall; his direction of the choir is another sign of an "obvious leader."
It depends on how you define better. Ralph is moral. Jack is savage, savvy and strong, he gets the upper hand. So is leadership defined by who is better qualified to lead? The success the leader has, or the moral character that the leader exemplifies?
Clearly, Jack taps into an evil force while the boys are on the island, while Ralph, though seriously tempted, resists.
Although Jack's actions help the boys physically survive, Ralph is my choice for the better leader.