Now that I have slept on it, I will answer OP's question. Understand though that there is a respectful way to ask such a question and that this was not it.
The top candidate in my opinion is Genyo Takeda. In the wake of Iwata's passing, he along with Miyamoto are the acting Representative Directors. I'm not entirely clear on what that means, but my understanding is that until a successor is chosen, the tasks that would usually fall to Iwata are now their responsibility. My hope is that someone with a similar background to Iwata is chosen; Takeda has been the general manager of Nintendo's integrated research division for over 30 years, and is also the creator of Punch-Out!!, Star Tropics, and a handful of other games. He is credited with the invention of the battery save system first used in The Legend of Zelda NES cartridges and with the invention of the analog stick. I would much rather continue to see someone who got their start working on games and picked up the business know-how through experience rather than the other way around.
Iwata will be succeeded, but he cannot be replaced. He was more than a decision-maker at Nintendo. His personality and sense of humor permeated the entire company. He was close friends with his fellow managers, with many of the designers working at Nintendo's development studios, even with game makers outside of Nintendo.
Those hoping for a big shake-up at the top that results in significant changes to Nintendo's M.O. are going to be disappointed. They will appoint someone who believes in Iwata's vision for the industry, and embrace that vision whole-heartedly.