Definetly Ozzy, so overrated. I like Black Sabbath, though not as much as some of their contemporaries but they have some great songs with Ozzy as the frontman. War Pigs, paranoid, After Forever, Supernaut, NIB...when you think about it he was definetly the best Sabbath singer ever. By best I don't mean he was actually a better vocalist than Gillan or Dio who came afterwards, but he simply fit in their musical style best, the horror atmosphere, the eeryness, it was best accompanied by Ozzy's sick sounding voice.
Ozzy is not some great singer, never was, and all the fanboys of The Osbournes should realise that, however he has done a lot for Rock N Roll and Heavy metal, for that I'll always respect him.
Another example is David Gilmour from Pink Floyd. Now to set everything straight before someone gets a heart attack, I LOVE PINK FLOYD and I love Gilmour. But when you think about it, he really doesn't have a good voice, average at best. But Floyd is such an improvising and artistic band, they never really did rely on their vocals that much.
Another example, though not one of such magnitude, is Chris Cornell, former Audioslave and even former Soundgarden. In Soundgarden he was good but in Audioslave he was so awful, his voice deteriorated so much. I get the feeling like he's shitting himself. Awfull.
Oh and I almost forgot, Blaze Bailey, former Iron Maiden. Jesus, I know Dickinson's shoes were hard to fill but come on! That was simply breathtakingly horrible. I'm mostly sad because he ruined some great songs written by them in his era, The Clansman and Sign Of The Cross for example.