#1 Bret "The Hitman" Hart (regretted joining WCW)
Bret Hart had an incredible career in WWE and was a part of the main event picture for years on end. He was essentially the face of the company in the mid-90s, and had one of the greatest rivalries in WWE history with Shawn Michaels. Unfortunately, he wasn't a fan of Shawn in a real-life. Hart bagged a contract with WCW in 1997 and made it clear to Vince McMahon that he didn't want to drop the WWE title to Michaels at Survivor Series.
This led to Vince double-crossing Hart in Montreal. In an appearance on WWE Backstage, Hart stated that he regretted his decision to join WCW. When Booker T asked if he would like to change things if he got another chance. here is what Hart had to say in response:
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"I think I figure out a way to stay in WWE. Knowing what I know about WCW, I'm sorry I ever went there."
Hart was a 2-time WCW World Champion and won several other titles, but wasn't used by the company to the best of his abilities. Hart also suffered a career-ending injury at the hands of Goldberg in WCW.