Rob Halford, without a shadow of a doubt. I like Ozzy and all, he's a great character, but musically he isn't that good I think, I think even RH performs Black Sabbath songs better than him!
Halford also changed the face of metal with the whole S&M leather and studs carry on (well, him and Judas Priest bassist Ian Hill), and I think changed many attitudes by being the first big name to come out as gay, because, if I'm honest, metal never struck me as the most gay-friendly genre! Let's not forget the six-octave singing range and legendary songs like You've Got Another Thing Coming, Breaking the Law and United that he wrote!