Euron Greyjoy wins
Euron Greyjoy was the surprise MVP villain of Game of Thrones Season 7. A late addition to the Thrones ensemble, Euron now controls the Iron Fleet, which makes him an asset to Cersei. In exchange for helping Cersei win her wars, Euron has been promised that he can marry her. If Euron does marry Cersei, that makes him a King, and then anything that happened to House Lannister after that wouldn’t matter as House Greyjoy would control the Throne.
With the other armies marching North to fight the Night King, Euron is sailing to get the Gold Cloaks, a mercenary group in Essos who have supposedly never broken a contract. That doesn’t stop Euron from making a side deal with the armies in transit, or just making them a better offer once he realizes Jaime Lannister has abandoned his sister. We’re living in a Westeros where the dead have breached the wall, the Gold Cloaks breaking their first contract wouldn’t be the most unexpected thing given current apocalyptic circumstances. Then, Euron can choose to fight the Army of the Dead, or wait until they wipe out most of the mainland before going in for the Night King instant-kill. Embrace the Storm, Euron! Winning it all is within your grasp!