#1. Kevin Owens

Looking at the roster, Kevin Owens could be the perfect guy to defeat Brock Lesnar for the WWE Championship. Over the last few months, Owens has been on a roll, as he turned babyface and became one of the most over wrestlers on the roster. He fired Shane McMahon from SmackDown before moving to RAW during the draft.
Currently, he's involved in a program with Seth Rollins and the Authors of Pain. He cannot afford to lose it as he could drop a lot of his momentum. Once it ends, he can move on to Lesnar and pose a credible threat to the latter's reign. These two wrestlers have had very little interaction in the past, and it would feel fresh to see them collide.
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Owens also has most of the fans behind him, and a victory over Lesnar could be what he needs to become the face of the company.