Jack is the worst choice for leader because he leads like a dictator. He has little concern for the long-term well being of the group (being rescued) and spends most of the time hunting and intimidating others.
Piggy is a potential leader and he is the most logical and intellectual boy on the island. But at this point in his life, he is only a potential leader. This is mostly not his fault. Being awkward and fat, other boys have probably never given him the opportunity to lead. So, he's had no practice at it. In this hypothetical, with everyone being the same age as the boys in the novel, it is unlikely that the other boys would listen and follow Piggy. If they did listen to him, he would make an intelligent, fair, but perhaps weak leader.
Ralph is clearly the best choice. He is fair and honest. He delegates jobs well. Had the boys followed those delegations and kept the fire going, they would have been rescued much earlier. Ralph is not as intimidating as Jack. But, unlike Jack, Ralph does not have to lead by fear. The boys respect him because he is honest and has their best short-term and long-term goals in mind. After Ralph assures them that they will be rescued, he gains their respect:
The assembly was lifted toward safety by his words. They liked and now respected him. Spontaneously they began to clap and presently the platform was loud with applause.