Category: Pop music, broadly defined. Criteria: solely the ability to render beautiful music with the ability of one's voice, whether accompanied or not. Being a great songwriter (Kris Kristofferson) or dnacer (Michael Jackson) is irrelevant to the discussion at hand. Sorry that this list is so exclusively American, but that is due to my apalling ignorance of musicians from other countries.
1. Elvis Presley
2. Al Green
3. Sam Cooke
4. Roy Orbison
5. Frank Sinatra
6. Bill Monroe
7. Nat King Cole
8. Ray Charles
9. Conway Twitty
10. Ralph Stanley
11. John Conlee
12. Robert Plant
13. Keith Whitley
14. Freddy Mercury
15. Randy Travis