Molly Andrews sings a pretty good traditional ballad when she is of a mind to. Her Twa' Sisters variant that she got from Horton Barker is awfully good. She's my main contribution to your list. Got to agree with Burke about Sheila Kay Adams and Jean Ritchie. They're the best.

Doc Watson sings 'em and does the same great job he does with everything else.

Ralph Stanley does a traditional ballad from time to time.