The King ! The King ! The King ! The King ! The King !
I can't believe no one said Elvis.
Maybe not the best voice of all time (but damn good for the genre) and certainly not the best material, but no one, I mean no one, could sing like Elvis.
When I say "sing", I mean - convey passion and emotion. He had this wonderful, painful, soulful quality that was both unique and mesmerizing. IMHO, Sinatra was not even close in communicating feeling like Elvis. To me, Sinatra "articulated" the songs, Elvis breathed life into them.
I know some will scoff, but had he not been so flawed and had he had better management, no one would have ever approached his genius, even today. God truly did break the mold on that one!