I think you have to differentiate between the different vocal genres.
A good bluegrass singer is very different than a good opera singer than a good rock singer than a good Gospel singer than a good Broadway singer, etc.
If you’re looking for someone who can do it all, Elvis Presley. I don’t think he ever did opera, but he did all kinds of other music and did it well.
Also Ethel Waters, who did everything from Broadway to Billy Graham. Sammie Davis Jr, too. Marian Anderson. Myrtle Hall.
In fact, a lot of black singers are extremely versatile. I wonder if perhaps they had to be, to get work during times when color discrimination was legal in the U.S.
Aretha Franklin–in her prime, she was amazing. Not as versatile, IMO, as some of the singers above, but still a great voice.
I personally think that Johnny Cash had one of the most unique voices in history. To my knowledge, he’s one of the few singers in both the Rock and the Country Music Halls of Fame.
Also Roy Orbison-what a great voice.
I like Patsy Cline, too, and her modern-day reincarnation, K.D. Lang. Their voices are like silk.
I love Renee Fleming and think that she is a great diva of the opera.
I like Taylor Hicks–great R and B voice. Lots of soul.
I LOVE TONY BENNETT! To me, his voice sounds like Rocky Road ice cream–sweet and cool! He’s better than ever.