4: Samoa Joe

2018 was not exactly the best year for Samoa Joe.
Despite dominating on SmackDown Live, Joe constantly came up short in title matches, losing multiple times to AJ Styles in enjoyable though predictable bouts.
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A former Impact and NXT Champion, Joe has had championship success everywhere except SmackDown, something he'll be hoping to rectify in 2019.
If he does, it'd be a huge win for Joe, though the company would have to tread carefully with a heel Vs. heel dynamic.
Although, if WWE wanted to turn Bryan face, or even turn Joe face instead, having the Samoan Submission machine destroy Bryan could be the way to do it.
Whichever way the company goes, hopefully 2019 will be the year where Samoa Joe takes his rightful spot on top of the SmackDown Live food chain.