Judy Collins. Peter Yarrow's daughter, Bethany. Jo Stafford (must've been mentioned).
Jo Mapes when she was younger. Audra MacDonald. Renee Fleming. Ella, Sassy and Billie (though Billie never had a legit voice), Russ Columbo, Dick Powell, Cesare Siepi (who you've probably never heard of but was one of the leading bass opera singers in the world and a contemporary of Jussi Bjorling), George London, Pavarotti of course but Fisher-Diskau (sp?) was the master of Schubert Lieder. Bidu Saou (sp?) Villa Lobos interpreter,
Netania Devrath, Mahalia,

Some would debate Ethel Merman but Gershwin told her not to take voice lessons.
If you ever saw her live on the Broadway stage, you wouldn't forget her.

Now for the tradition side:

Jean Ritchie, Texas Gladden, Jeannie Robertson, Margaret Barry, Nimrod Workman, Horton Barker, and Bess Hawes (though not many heard her sing solo), Hally Wood, Vera Hall,
Josh White, Burl Ives, Lloyd and McColl, Silly Sisters, Liam and the Clancys with Tommy Makem and his mom, Sara, Bill Chipman (Art Theime knows) and for that matter, Art Theime, Shoshana Damari, Martha Schlamme (though trained knew the Yiddish feel),
Alan Mills (from Canada), the blues shouters: Big Bill, Son House, Lightning,afro-folk Mance,Mississippi John, South African Miriam Makeba, Amalia Rodriguez, Germaine Montero, Cynthia Gooding, Uncle Dave, Doc Boggs, Mercedes Sosa, Piaf? (some would argue about this one), ..........................

If you don't know these folks you should if you think you know folk music.

Then the pop side: Whitney, Diana, Gladys Knight, Smokey, Barry Manilow (yes he can sing), Andy Williams, Nat King, Louis, Johnny Mathis, Celine Dione, Mariah Carey,Tony Bennett, Dick Haymes, Connie Francis, Rosemary Clooney, Anne Murray,
Ronnie Gilbert, Holly Near, even Elvis could sing, throw Sinatra in there as stylist,
Frankie Valli, Ink Spots, Stevie Wonder, ..........................................