Hands down Ian Taylor. Twice turned down the opportunity to play for Barbados as he wanted to play for England but never did, absolute travesty. Always the complete professional, started life as a forklift truck driver which probably explains the difference between him and some of these petulant players who are handed careers on a plate.
Not sure he had unrealised talent. He played well over 200 games for Villa and was (I think) voted their player of the 90s