#1 Bray Wyatt
Seth Rollins was victorious over Braun Strowman at Clash of Champions and will face a new challenger at Hell in a Cell. That challenger is Bray Wyatt's brand new alter-ego, The Fiend. The Fiend made himself noticed after attacking Rollins post-match at Clash of Champions.
Wyatt must win the title and there can be no debate about that. The Fiend is the most talked about Superstar in WWE today, is extremely over with the fans, and having him win the Universal Championship makes way too much sense. If he lost, then it would absolutely kill his momentum. He could come back from it, but if he is the hottest thing on the roster, just give him the championship and let him run with it.
The Fiend has already beat a former WWE Universal Champion (Finn Balor) and he is ready for his very first opportunity to carry the Universal Championship.