Brian Pillman
While Pillman wasn't necessarily a main-event player in either WCW or WWE, he was a key part of WCW in the late 80s and early 90s and in the WWF during a tough time of transition in the middle of the 90s.
He was integral to the progression of Stone Cold Steve Austin's WWE character and the two were able to build off of their success as the Hollywood Blondes during their time in WCW.
Charlotte Flair to return and wrestle a CURRENT CHAMPION?
Ric Flair and the WCW brass must have thought enough of him so much that they tried to relaunch a newer version of the Four Horsemen with Flair, Anderson, Pillman and Chris Benoit.
The moment when Austin showed up at Pillman's home on an episode of Raw and Pillman brandished a gun, is thought by many to have started the Attitude Era in WWF.
He was such a phenomenal athlete that he played in the NFL and fought Jyushin Liger on more than one occasion. Pillman might not have been a champion in WWF but he certainly made an impact wherever he went.