Freddy Mercury, Roy Orbison, & Elvis Presley are the first names that come to mind, but I'd also suggest that General Johnson (Showmen, Chairmen of The Board), Cornelius Harp (Marcels), Tony Williams (Platters) and a handful of other Doo Wop singers deserve consideration. The best of those guys - and there were quite a few - could really sing!
Marty
Marty