*Ronnie James Dio - I think he has the most powerful voice in metal/rock. Listen the Stargazer from Rainbow Rising album. he is awesome.
*Geddy Lee - The first time I heard him was on Rush's debut album. I could not stand him at the time. I took some time, but i grew to love that voice.
And others...
*Ozzy
*Freddy Mercury
*Robert Plant
*John Lennon
*Dennis De Young (Styx)
*Some guy from Kansas (Carry on wayward son, Steve Walsh?)