A few thoughts...
Seems to be a heavily based male singer focus so I'll throw out a few of the ladies
Patsy Cline
Dolly Parton
Linda Ronstadt
Also, as much as I'm a fan of George S, I don't get why so many people are listing him as one of the best singers. His range is limited, if you listen to him live he drifts at times, and there's nothing that special about his tone/phrasing. On the other hand, I think he's had some great singles (early moreso than later). In fact, the person who wrote some of his biggest hits (The Chair, Ocean Front Property, etc.) is from Knoxville.
The possum, by far, is/was one of the best pure male singers in country music.