Vince Neil - Motley Crue & Joe Elliott of Def Leppard are both sadly woeful when it comes to live 'singing'. Last few times I saw Vince he only just managed to squeal about every third word of most of the songs and spent a long time with his mike held out towards the audience for them to sing it for him.

Those two I'd claim are well beyond subjective - they just can't cut it any more.

On a more subjective side though there's people like Roddy Stone who 'sings' for Viking Skull. His voice is definitely an acquired taste, but for me he is very very good at what he does. Live he comes very close to replicating what he sounds like on the albums.

There is a definite difference between having a pleasing tone and being capable of hitting the notes you're going for - but then even a 'good' singer who is pitch perfect within their range will sound awful if they stray outside that range.