#7 Sami Zayn
In theory, booking Sami Zayn's main roster gimmick should have been fairly straightforward. The man is clearly a loveable babyface underdog who gains just as much fan support from losing matches as he does winning them. This basic approach did very well for him down on NXT.
His eventual victory over Adrian Neville, for example, was one of the best-booked matches in the faction's history because it was something the fans wanted, but were not sure they were going to get.
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For some reason, this has not translated well onto the main roster, either on RAW or Smackdown Live. The problem seems to be that in terms of the evolution of the Sami Zayn character, not much has happened since his days on the Indies as 'El Generico'.
Rather than properly test Sami out against the best the WWE has to offer, his time on both main roster shows has mainly revolved around his rivalry/friendship with Kevin Owens. And while this is a compelling story, fans grew tired of seeing it play out long before the writers did.
Now that Zayn has turned heel and aligned himself with his former nemesis, it seems his career will again be determined by his proximity to Owens, meaning even a heel turn has done little to allow the man to grow on his own.
Even the strongest Zayn defenders are now starting to admit that there is very little hope of him ever holding one of the main championship belts. His career has definitely reached a plateau, but we will hopefully get to see him in the mid card for quite a few years to come.