My favorite singers (in loose order) are:

Mark Sandman (Morphine)
Scott Weiland (STP)
Brandon Boyd (Incubus)
Angelo Moore (Fishbone)
Selene Vigil (Seven Year B itch... from purr to snarl like nothing)
Les Claypool (do I really need to say this one?)

Their's more, but my brain isn't working right now.