Dear raging dolt, the question was in fact about voice, i.e. timbre, color, THE sound...and not about vocalization, i.e. power, range, control.
As such, I think the number of rock singers with truly great voices are few. Granted, rock doesn't demand "pretty" voices or pliable vocalists. In fact, ability can be a detriment...see Geoff Tate, King Diamond, early Zeppelin blues massacres.
But just for the fun of it. To attempt to make the game a little less subjective, I'm thinking what rock singers could sing the classic standards, (Sinatra, Andy Williams, etc.) or say, "The First Time Ever I Saw Your Face" without making grandma cringe.
Dio, for sure. Both in his falsetto and modal range.
Rod Stewart, for character as much as tonal quality.
James Dewar, especially his falsetto, which is saying a lot for a man.
Van Morrison
Halford, not sure. Nice lower-mids. His falsetto is great when screaming, but we're not screaming here. He's one of the many right on the cusp.
A few more. I'm tired