For me it's a tie between four guys, MVP, Mr.Kennedy, Muhammed Hassan, and Curt Hennig.
First off I'm gonna start with Curt Hennig, he had a perfect gimmick, a perfect look, perfect mic skills, and amazing talent. But his big push suddenly stopped when he had an injury sometime in the early 1990's, and he was forced to be Ric Flair's.. what was it? assistant? let me just call him Flair's A-RI with all do respect to both him and Alex Riley I think his relationship with Miz and Cole was based off Perfect's relationship with Flair and Heenan. So when he recovers they turn him into a babyface, he had a decent run as a face teaming with Macho Man to take on Flair and Razor Ramon, But he never got his ME push cause of all the guys who were overshadowing him, and rumor is he was supposed to win the 1990 royal rumble, but never did cause of Hulk Hogan and his game of politics, so he never was a ME player.
And when you look it Muhammad Hassan he was one of the most despised wrestlers in WWE history, he was a heat magnet, he had talent, he had a great look, but it all got ruined when for some reason(I'm blaming Michael Hayes for this) he was given a terrorist angle, and with the worst timing possible for it, he eventually got written off TV and then got released.
Then you had MVP, the guy could talk, the guy could wrestle, and he was over with the crowd by the time of his feud-partnership with Matt Hardy, which bring me to the entertaining programs he had with Kane and Hardy, he was destined for, though eventually he became a babyface and it ruined his career, and he eventually became the guy who put over the younger talent like The Miz and Jack Swagger.
Then you had Mr.Kennedy..Kennedy.., he was the most entertaining part of Smackdown! back at around 2006-2007, he was placed in a feud with The Undertaker similar to MVP and his feud with Kane, and he actually beat The Undertaker once thanks to MVP, and the WWE were so high on him that you just knew that hes gonna make it big, unfortunately Randy Orton ruined that for him and ruined Kofi Kingston too a year later, so it's a shame how he ended up and considering he's in TNA now I don't even know how he's doing there but that company went downhill ever since Hulk Hogan came in.
Honorable Mentions : Heidenreich, Sean O'Haire, Carlito.
Now I would mention Drew McIntyre and Chris Masters, but their careers aren't over yet, so here's to hoping they improve their mic and in ring skills and make it big one day.
I agree 100% on Hassan and Henning. People complain about Cena now, Hogan really held a lot of great heels back in my opinion. Mr. Perfect and Ted DiBiase were the two best heels of all time and both should have had a run with the title.
Muhammad Hassan could have been a great heel if they would have been a little more careful with his booking. There was no need for the whole "terrorist" angle and Michael Hayes should have been fired on the spot. If you would have just played him as an Arab-American who was pissed for being profiled and now he was denouncing America it would have worked. His music pissed people off, his look pissed people off, he was good on the mic and good in the ring. Personally I think he should have been given the U.S. title as soon as he moved to Smack Down, that would have went so well with his gimmick.
My others are:
Magnum T.A.: I mean this one is obvious, he was on his way to super stardom before his accident.
Simon Diamond: I seen this guy in ECW come out at the begining of a PPV and cut a promo and thought to myself this guy just has "it" I don't know why he never amounted to anything, but I thought he was great.
Mr. Kennedy: If he would have got that Vince's son angle, I think he would have really ran with it and became a top draw. Great mic skills, great in ring skills and he just had this drive that I don't see anymore now that he is in TNA.
Johnny "The Bull" Stamboli: I'm not going to lie I was a mark for the Mamalukes in WCW and the new F.B.I. in WWE. I thought there was a lot of work to be done with him, but I thought he could have at least been a good mid- carder.
Sean O' Haire: He had a great look, agile for a big man, but he just couldn't get over and I could never figure out why. Hell they gave him Piper as a mouthpiece what more do you need?
Monty Brown: I liked this guy in early TNA, I thought he had a lot of upside to him, former NFL player, big, quick, could put on a decent match. Then when he went to WWE as Marcus Cor Von it was like a completely different guy.
Disco Inferno: I would have loved to have seen this guy in WWE as a wrestler or as a manager. I remember way back in the day they said Honky Tonk Man was supposed to bring him in and manage him. I thought he was good on the mic, it was kind of corny but I actually liked his gimmick and he could put on a good match.
Colt Cabana: I know he is still a big name in ROH, but I think WWE really missed the boat on this guy. He may not have the "WWE physique" but the guy is golden on the mic. His shows on WWE.com were hilarious, he is a wrestling trivia book, and he can work. I would loved to have seen him and C.M. Punk go at it during the SES angle. I think he would be a gem on commentary as well.
Honorable Mentions: Matt Bentley, Petey Williams, Adam Bomb, Nova, Prince Iukea