I'd agree with many of the above names. I don't think there is one best/purest/whatever singer. I don't even have a 'favourite' singer, but there are a lot of really good ones around. Some people I'm listening to a lot lately are Karine Polwarth (sings with Scottish bands Malinky & MacAlias and was with the Battlefield Band for a while) and Jane & Amanda Threlfell from England. But how could you say they were better than Emylou, or Niamh Parsons or Allison Krauss or Kate Rusby or Sandy Denny or Linda Thompson or Dolores Keane or Mary Black or Mairead Ni Domhnall or Mairead Ni Oistin or Mairead Ni Mhaonaigh or ....

Music isn't supposed to be a competition - go out and enjoy them all.