Just to add to some of the above and agree with many but I like the following Jazz - Billie Holiday (both verve and columbia recordings, probably the most important singer of the 20th century after Sinatra), Dinah Washington (particularly the early mercury recoardings), Ella Fitzgerald (the verve songbooks), Sarah Vaughn, Nina Simone (although her output varies, generally the earlier recordings are best),Betty Carter and Shirley Horn; in pop/folk - Eva Cassidy (one of the best you've probably never heard), Sandy Denny (one of my all time favorites), early Joni Mitchell (the Blue album and before) and early Judy Collins (albums from the 60's and 70's for the most part). Hope the above is helpfull.
