#1 Mr. Kennedy
Mr. Kennedy/Anderson was someone who many within WWE, including Vince McMahon himself, looked at to be the future of WWE, but it wasn't meant to be.
Kennedy debuted for WWE in 2005 and showed immediate potential. He was a very talented performer, very capable of putting on a solid match, but his true niche was his skills on the microphone. Upon arriving in WWE, he quickly became the best talker in the company and the best talker since The Rock.
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He had a good run on the mid-card. He held the WWE United States Championship and scored victories over The Undertaker, Batista, Booker T, Rey Mysterio, Chris Benoit and more, all within a very short space of time.
WWE had big plans for him being in the main event. It almost seemed like the World Championship was guaranteed to be his as he won the Money in the Bank contract at WrestleMania 23.
However, during his run with WWE, Kennedy would be plagued with injuries, and not just any kind of injuries, but back to back ones that made him miss considerable amounts of time, and they would force him to give up his Money in The Bank shot.
While Vince McMahon still thought rather highly of him upon his return from his 2007 injury and planned for him to be revealed as his 'illegitimate son', it was apparently Triple H who reportedly got in Vince's ear, convincing him that Kennedy wasn't the right guy to lead the company in the future.
Kennedy spent the next 2 years having various injures and wrestling in the mid-card, before he was fired by WWE after being deemed reckless in the ring by John Cena and Randy Orton. Kennedy resurfaced in TNA in 2010 and would be featured prominently in the main event scene and become a 2 time TNA World Heavyweight Champion during his on and off four year run with the company.
Still though, I think Mr. Kennedy is easily the biggest missed opportunity to be a huge star and WWE Champion over the last number of years.