Agree on Cline and Orbison. Some favorites of mine were Dean Ford of The Marmalade, Johnny Maestro of The Crests and later The Brooklyn Bridge, Jay Black of Jay and the Americans, Burton Cummings of the Guess Who, etc.
I won't put all the links on here, but if you like a great singing voice, go to youtube and check out Brooklyn Bridge--Blessed is the Rain, The Worst That Could Happen, You'll Never Walk Alone. Burton Cummings and Stand Tall. The Marmalade-Reflections of My Life. Dean Ford-In Dreams, Jay and the Americans-Cara Mia, This Magic Moment, Walking in the Rain, Crying..