Jack is the antagonist because he stands in the way of Ralph's two goals of Parliamentary island rule and building a fire.
But let's not draw lines too quickly. Both boys are leaders, both are strong (physically and in character), and the other boys look up to both. We get the feeling that Jack could have been our protagonist—if it weren't for his lust for power, blood, violence, and cruelty.
Sure, in the literal sense, you could say the beast is actually just the dead man in the parachute, which would make the antagonist the war that killed him. Or you could say, as Piggy does, that the beast is everyone's fear—which makes fear the antagonist. Or you could jump in Simon's boat and say that the beast is "only us" (5.195), i.e., the darker side of human nature. And if that's true, then maybe Jack isn't the antagonist after all: he can't help being human.