#4 Neville
Neville, now known as PAC in All Elite Wrestling, was once considered by many to be a major player for WWE in the future. During his time in NXT, he won the NXT Championship and had several incredible rivalries and matches.
After being called up to the main roster, Neville had a decent showing but could never make it big. WWE moved him to the Cruiserweight division, where he turned heel and had a great run as the Cruiserweight Champion. But it was in October 2017, when Neville left WWE, with the reason being dissatisfaction with his place in the company.
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But his WWE exit was pretty controversial as before walking out of the company, Neville reportedly engaged in a "crazy shouting match" with WWE Chairman, Vince McMahon. He even went to the extent of flipping out on Vince before leaving.
WWE then refused to grant him an early release, freezing his contract and making him wait for almost a year before his official exit in August 2018. In January 2019, Neville, now known as PAC, joined AEW - the new promotion of Cody Rhodes and the Elite.