Lena Horne, without a doubt, oh wait a minute, Mary Wells was there too. This is a most interesting thread. Eva Cassidy might have had the most beautiful voice or been the best non-operatic singer (certainly one of my 2 or 3 favorites) and you couldnt help falling in love with her singing, but she wasn't exactly a looker. There were so many in the late 60s early 70s. The young Emmylou Harris was very attractive. Martha Davis of the Motels. Barbara Keith, who now in her early 50s sings with her husband and stepson in the band called the Stone Coyotes. Marilyn Monroe did make a recording, so she qualifies. Currently, Shania Twain is hard to take your eyes off of. And on the other side of thte coin, the least appealing, totally unattractive and talentless "singer" around is Brittney Spears. I remember seeing in a news story a while back that a Brittney Spears look-alike contest was won by a boy.
