#1 Matt Riddle
Matt Riddle is the best. He just is. You can't debate this. Since joining NXT he has become beloved by the fans for just being himself. His character is just him. He's a fun, chilled out dude who also obliterates his opponents due to his former MMA background.
For a guy who has as much natural charisma as him, it is almost impossible to screw him up on the main roster. While that could happen, his MMA background means that Vince could pull the trigger on him and make him the world champion.
The fact that he is so different from everyone else on the roster makes it seem more likely than most people to be pushed.
If the crowd stay behind him in NXT and Vince McMahon likes his babyface character, maybe one day we will see him win the WWE title. And maybe retire Brock Lesnar. But that would just be a bonus.