King's Landing is the High Sparrow's town now.
Will that matter in the long run? Probably not. Ice zombies and dragons take precedence. King's Landing just isn't that important anymore.
Still, it's impressive that some nobody cobbler allegedly woke up hungover after a party, became disillusioned with the world, and went on to accrue so much influence that he's now basically controlling the government. He started from the bottom and now he's here, polluting Tommen's naive mind.
He's capable of bold power grabs and subtle persuasion. He's able to amass a large group of devoted Faith Militant followers, whose power lies in their numbers. He knows how to play both the powerful and the powerless.
He may be sinister or sincere. At the end of the day, it doesn't even matter. The fact is he's got mad leadership abilities. That's why he's number one on this list.