Black Dahlia
A Seasoned Witch
Pfff... that's easy: Paul Rodgers
As far as quality of the singing goes there would be many up for the mantle. I'm not saying he's the best, but I would throw David Gilmour into the mix for the pure variety he's produced over the years. When he first joined Pink Floyd he was known for being able to mimic other singers, and that's what he did when they were touring in '68/'69 - mimic Syd's vocals. Since then he's demonstrated the ability to change his vocal style to fit the song. It could be a fairly harsh delivery (Young Lust, Time, Money) or a smoother one (Fat Old Sun, Comfortably Numb).
He's obviously more renowned as a guitarist, but I think he's been under-rated as a vocalist.