#2 Finn Balor
Why not end it where it all began?
Seriously, why not take it full circle and have The Fiend face off against The Demon King? Not only would this be a great excuse for Balor to summon The demon again in order to take on The Fiend, it could also be the start of him doing it more often.
Another reason why Finn Balor is the perfect person to finally defeat The Fiend is the fact that he is one of the only other Superstars on the roster that has a supernatural gimmick. This will ultimately allow WWE to protect The Fiend in defeat and explain away the loss in a way that they would otherwise not be able to pull off with any other Superstar.
For example, if someone like John Cena were to defeat Wyatt straight up, it could come across as cartoonish and may even be seen as completely unbelievable, as the aging star probably doesn't have enough gas in the tank to bring a strong enough fight to the most powerful and feared men on the roster. Not to mention that Cena would single handedly be responsible for killing WWE's biggest creation in several years and could face a lot of fan backlash, which would be a repeat of what happened half a decade ago at WrestleMania XXX.
Compared to The Demon King , who has a supernatural element to his character, and the story becomes a totally different beast and could lead to a rivalry that continues on-and-off for many years to come.