Was better: Eric Young

Similar to EC3, Eric Young was a big star in TNA. His move to WWE was a decent one too in NXT but just could not replicate the same on the main roster.
In TNA, Young won several championships, including the Heavyweight championship and the X Division championship. However, in WWE, he won the NXT tag-team championship once. In April 2020, he was released by the company along with several other Superstars in the mass exodus.
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Became better: Chris Jericho

Chris Jericho is easily one of the best wrestlers ever but he would have never managed to reach the heights if he had not joined WWE. The 49-year-old was a part of several promotions including WCW and NJPW before moving to WWE.
When Jericho switched to WWE in 1999, he hyped up his debut so much and it kept the WWE Universe glued. He ended up winning almost all the WWE titles but surprisingly never won the Royal Rumble or the Money in the Bank briefcase, the latter of which was his brainchild. Now that he is with AEW, a return to WWE seems unlikely to happen for the foreseeable future.