For me he's always been about the image and the presence (charisma) more so than being someone with great vocal talent. Personally I think he is a terrible singer, but his voice perfectly fits his music and personality...

I prefer DIO, Ian gillan or Ray Gillian (that right?), Glenn Hughes as Black Sabbath singers although I'm aware that their contributions to BS were not as well valued as the classic lineup. That said "Heaven and Hell" is a really good album!