The answer to the question of who is the best leader in Lord of the Flies depends heavily upon defining what exactly is meant by the "best leader." If "best" means the most successful leader, then Jack was by far and away the best leader. Although his tactics were brutal and blood-thirsty, Jack commanded the respect of his tribe, becoming much more powerful than Ralph ever was. Jack used fear and intimidation in order to make his tribe follow his commands and do his bidding--something that Ralph was never able to do successfully.
However, if "best" leader refers to someone who is fair and looks out for everyone's best interests, then Ralph was definitely the better leader. Ralph had great ideas as a leader--ideas that could benefit the entire tribe like building the huts for protection from the elements and keeping the signal fire burning for rescue. Ralph attempted to use power fairly and democratically, even setting up rules for the tribal meetings like holding the conch to have a turn speaking out loud. Ralph's leadership style was infinitely more civilized and fair-minded than Jacks, but in the end, this approach was not enough to for him to hold onto power against Jack's savage tactics.
I would say that Ralph was the best leader he was always taking charge, trying to control behavior on the island, and he knew they had to keep a fire going to get rescued and tried to comfort everybody. Unlike Jack where he didn't care much about the fire but he did have a group of hunters that got food he also segregated himself from the others creating his own tribe and in doing that also he killed two people and was out to kill Ralph. Roger and Jack also threatened and basically beat Sam and Erick to join his tribe. In my opinion Ralph would be the best leader. It could also best argued both ways as well.