Speaking about wasted potential I present the opposite case, a fantastic singer caught in a band that doesn't do him justice:
Eric Adams from Manowar. If you ignore their lyrics there are certainly songs that are great as far as the music is concerned. Eric is insanely talented and has delivered some of my absolute favorite performances in metal, but Manowar is just sooo goofy. It is what it is I guess, but just imagine him in a project with subject matter closer to what Maiden is doing for example.