I'm not really a metal person anymore, but I think there's an argument to be made for Mikael Åkerfeldt. Certainly if you look at influence and longevity, etc., it has to be one of Bruce, Halford or Dio. But if you put those things aside, I think there's an interesting argument for Åkerfeldt due to his versatility, and, honestly, the high quality of both his growls and his clean vocals.