Correct technique and bagwork. IMO nothing beats heavy bagwork for developing explosive power. All the muscles in the world wont give you a KO punch if you dont have correct technique and body mechanics. A punch great punch should start at your feet and end with your fist, the whole body plays a part.

Core strength is also important. Drop stepping is also a good way of delivering power in your punches. Dempsey used to drop step into his punches and look at the power he used to get and he wasn't the biggest chap in the world.