From left field - Randy Sparks (before he formed the New Christy Minstrels) was a very credible balladeer.
Rusty Draper (made a hit with Willie Nelson's "Night Life" years ago)
I always thought that country singer Kathy Mattea had a near perfect voice for traditional folk music, had she been so inclined.
In pop, Sinatra always said that Vic Damone had the best pipes in the business. The unspoken inference was that he was a "technician" who lacked "soul." I prefer Tony Bennett, who has both.
Roberta Flack had both great pipes and soul. She might well have been very successful as a folk artist.