Jon Stewart of Slauter Xstroyes is the best singer in all metal. I don't care that he's a fifth-tier nobody; his voice successfully mixes the grit of Bruce Dickinson, the spontaneity of King Diamond, and the ear-piercingness of Ian Gillian into one wonderful master of traditional/power godliness.
Jon Oliva almost makes it, but towards the late 80's/early 90's his voice could get unpleasantly shrill and whiny at times. I won't deny him the claim of largest metal testicles circa Sirens through The Hall of the Mountain King, however.