Cersei Lannister: The Queen Regent
The power behind the throne for two kings and counting, Cersei successfully ended her drunken husband Robert's rule, then fought like hell against her brother Tyrion to protect her incest-born son Joffrey. (Of course, Tyrion was the one with her shitty kid's best interests at heart, but don't tell her that.) Now Tyrion's been sidelined just as effectively, and nearly as permanently, as Ned Stark in Season One, leaving Cersei to rule the roost. Even if her father Tywin proves just as uncooperative as his dwarf son, Cersei's still perhaps the only person her son the King still listens to, and in an absolute monarchy, a little goes a long way.