I'm going to toss another name in here even though WCW built him to begin with...
Chris Kanyon
It seems that everyone who knew him or worked with him has a story about how unfairly Kanyon was treated. I can't think of another person that has so many peers who will complain about how much that person's talents were squandered. Kanyon may have had a slight lisp, and when it came down to it I'm sure his homosexuality hurt him too, but in every other aspect he should have been one of the top stars in WCW.
Kanyon could be compared to Dolph Ziggler these days if not for the fact that Ziggler has been given numerous opportunities to put on big money matches in his career. Kanyon was relegated to the midcard and treated as if he was lucky to be there. To be fair, WWE was just as bad when it came to this, but in a time when WCW was starving for new talents to push, Kanyon was never among them.
We talk a lot about Vince McMahon's infatuation with size but WCW may have been an even bigger culprit of this bias. With their backs against the wall, they pushed Scott Steiner and Booker T, both fairly large guys. Now imagine if they didn't implement the glass ceiling quite like they did and they had a few other names milling around the top of the card with Steiner and Booker. For example, if WCW had a main event scene of guys like DDP, Jeff Jarrett, Goldberg, and Sting mixed in with Steiner, Booker, Lance Storm, Mike Awesome, Chris Kanyon and, hell, even Billy Kidman then WCW would have had that shot of youth that they needed to stay relevant.
Instead, Kanyon was relegated to DDP-lite, Storm reached the glass ceiling and never got back up, Awesome was sabotaged in every imaginable way, Kidman may be the first person to ever have their career ruined by WINNING a match against a legend, and even Booker just kind of had the World Title thrown on him with no rhyme or reason. If I was a business executive, looking at how much money WCW was bleeding, and seeing no future chance of drawing money, then I'd have closed them down too.