Remember seeing John Paul Kissock in a reserve game against Reading and he was head and shoulders above everyone, looked the real deal. A certain Victor Anichebe looked woeful.
Other than that probably Vaughan and maybe Gosling. Never thought Rodwell was much cop myself.
Rodwell was bang average. Had he been another nationality there wouldn't have been a word about him. Deulofeu for example is a far more gifted player. Rodwell made that one run (unopposed) and struck the ball into the corner of the net and the hype machine took off. He did little else. He wouldn't have been a regular here had we had money to spend at the time. Man City threw money around at anyone that moved. They soon realised they had been had.
Very few players don't fulfill their potential. They generally find their level. Would only say a player didn't achieve their potential if they looked great at the highest level then suddenly dropped from the level they had set. Serious injury, mental breakdown, or moving to a club that ask too much of a player are probably the only reasons for not fulfilling potential, and those things aren't that common. Michael Owen would definitely be someone that didn't fulfill his potential and that was due to injury.
Jeffers has been mentioned. He possibly could have done more with his career. The problem is he went to Arsenal and the gulf in technical ability between him and Henry, Kanu, Pires and co. was vast. Wenger wanted a 'fox in the box', someone to get on the end of stuff, but that was never going to be enough for that Arsenal side. He looked like an imposter and was never equipped to fit in with their fluid attacking style. Jeffers at best was a UEFA Cup level striker.
Richard Dunne had an excellent career for his ability.
Anichebe was very limited technique wise, there are plenty of more gifted forwards than him in the lower leagues. His physicality gave him a better career than more skilful players. If anything he has over achieved and made a good living.
Vaughan is another lower league level forward.
JP Kissock we never saw hit any kind of level so it's impossible to know if he could have done more.
Ross Barkley is only youth player from the club who I would say has unfulfilled his potential thus far. He has shown snippets of real quality. The serious injuries and the style of play here with shoddy managers didn't help his development, and it could be too late for him.