This list already has a ton of pop & jazz singers, but I'd add Kurt Elling, those of you that ever heard him sing "Ballad of the Sad Young Men" should agree.
Have I missed something? Where is Steve Goodman, Keb' Mo', Bonnie Raitt, Nic Jones, Paul Brady, and/or Kate Rusby. They often float to the top of my playlists.
These days, I'm being amazed by Martin Simpson, he has matured so much as a songwriter and a singer. Must be tough that these talents have been overshadowed by his career as one of the world's greatest guitarists.