Dfrigovt that was Jim Dandy Mangrum, though I'm not sure of the spelling of the last name. I saw them in concert in 1978 opening for Blue Oyster Cult.
I'm pleasantly surprised to see several mentions of Ronnie James Dio. He is definately the best singer in heavy metal, and possibly in Rock and Roll! There are many others that I like, but are not really great singers, but that's why there are so many different answers here. Although I have to wonder if the guy who suggested Bon Scott was serious or still under the influence???
I still like Ian Andersons voice. I like to listen to Bob Dylan, but I'm not sure that makes him a good singer!?!