I think most of mine mine have already been mentioned, but for me one stands above all the rest - Mr Ronnie James Dio. To the best of my knowledge there's nothing this guy can't do. Other favourites also include (in rouhgly ascending order): -

Bruce Dickinson
Robert Plant
Hansii Kursch
Geoff Tate (Queensrÿche)

Yes, i am a little metal-orientated...