AJ Styles

When AJ Styles signed with the WWE, a lot of people were hesitant with how he would be booked in the company. He had a good run in his debut Royal Rumble in 2016 and had early feuds with the Miz and Chris Jericho. Although he lost to Jericho at Wrestlemania, he immediately challenged Roman Reigns for the WWE Championship.
And when the brand split occurred, he became one of the top players on Smackdown. He eventually took the WWE Championship from Dean Ambrose and was involved in the main event throughout his time without the belt. He had a marquee match against Shane McMahon and eventually turned face again after shaking McMahon's hand after Wrestlemania 33.
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Since Jinder Mahal had won the WWE Championship off of Randy Orton shortly thereafter, a new face challenger was needed. At first it was Shinsuke Nakamura, and then Styles once again. In a shock move before 2017's Survivor Series, Styles won the title off of Mahal. The win catapulted him into a match with Brock Lesnar at Survivor Series. The match gave Lesnar one of his best matches in WWE since returning in 2013.
Styles held the belt for 370 days until he lost the title to Daniel Bryan on the Smackdown before this year's Survivor Series. During his record-setting reign, he battled Nakamura, Rusev and Samoa Joe. He carried the Smackdown men's division on his back for most of 2018 and is due for a bit of a break going forward.
Whatever he does forward, hopefully he can win the belt again either on Smackdown or Raw. It would be foolish for WWE not to re-sign him as he's proven that he's one of the best wrestlers in the world.