#1 "The Fiend" Bray Wyatt
"The Fiend" Bray Wyatt is one of the most dominant forces in WWE today. The Fiend has a twisted personality. He never forgets his former opponents, and seeks revenge against them.
From the start, The Fiend has been hunting down opponents who stood in his way before, like Daniel Bryan, Finn Balor and Seth Rollins. His victims suffer a change in personality after feuding with him. The Fiend's most recent victim was John Cena, whom he defeated at WrestleMania 36 in a bizarre Firefly Fun House match.
Bray Wyatt and Roman Reigns had an intense rivalry in 2015, which started when Wyatt cost Reigns the Money in the Bank Ladder match. They continued to feud for the most part of 2015, which came to an end when Reigns defeated him in a Hell In A Cell match later that year.
Since The Fiend mostly seeks revenge upon his former foes, it's only a matter of time till he comes after Roman Reigns, given the long history between the two men. Prior to the Super ShowDown PPV this year, Reigns stated that he would rather face The Fiend than Goldberg at WrestleMania 36.
Reigns and The Fiend are currently SmackDown's top Superstars. Once the two of them cross paths, things will only get more intense, given how well the two of them know each other. A feud between them has a very high possibility of happening.