I still like his voice on the later records, but I wouldn't dispute that he sounded better earlier on and that he could be obnoxious live.
My pick for "Most Ruined Hard Rock Voice" would be Ian Gillan. I don't know anything about the Ian Gillan Band/Gillan phases of his career but by the time he joined Black Sabbath I do not like his singing at all. Brian Johnson's voice was very inconsistent live and sounded pretty terrible on the '90s studio albums, but it improved some after he had throat surgery. Also, no matter how pathetic it sounds on Ballbreaker it doesn't grate on me like Gillen's later vocals do.