Neil Cybart, Above Avalon, makes the case for Jeff Williams, Apple’s SVP of Operations, as Tim Cook’s successor someday:
One thing became abundantly clear after analyzing Apple's recent earnings report: Jeff Williams is doing a phenomenal job. As senior vice president of Operations, Williams is tasked with making sure the Apple machine is well-oiled and in tip-top shape, not only capable of producing more than 100 million iOS devices in a quarter, but building flexibility into the system to handle annual hardware updates that would make most hardware companies quiver with fear. I considered Jeff Williams as Tim Cook's successor before Cook finished his first day as CEO, and I feel even more confident about that today. Regardless of what the future brings, people need to start watching Jeff Williams because he is executing at levels that few are able to achieve.
Related: Jeff's bio on Apple's site - link