None.
There is no one in the C suite that is competent to be the CEO (and that includes the current CEO).
The next CEO needs to be a product - not numbers - visionary. Apple's lineup, across the board, is stale and unimaginative. That's what happens when the COO takes over a product-based company.
The chances of it happening, of course, are exactly zero - the kind of person who could do it is probably stuck somewhere in low level VP or "senior director" hell, and Apple's board has shown zero courage, so the odds of Apple placing someone at the top who could actually reboot the company are non-existent.