Tyrion Lannister

As hand to the queen, he will be in place to take over if something should happen.

From early on Tyrion has been in the middle of the action. With connections to Cersei, Daenerys, Jon, and several other powerful characters, he is in a good position to take control.

Tyrion has been looked down on his whole life, but he is cunning, and that may place him in line for the throne.