I've seen Lyle Lovett, he is definitely on the A list, as is Tom Waits.

Is it still worth seeing Doc? If so, I'd love to still see him.

Dixie Chicks, I love their stuff, but their harmonies are kick ass & I don't think anybody out there right now has them beat.

Arlo, as I've never seen him.

Roseanne Cash--love that girl's voice.

Liam O'Flynn

Allison Kraus

Capercaillie

Wrigley Sisters

Tina Turner, but she insists she is retired now. I do hold out hope that she'll do a gig somewhere I might be...

Joni Mitchell

Ricky Scaggs

John Prine

And under the "if they ever did a reunion tour worth seeing" category:

The Kinks (if they've still got it)
Traffic (the orginals)
Rolling Stones

And if I thought their voices would still be good enough:

Everly Bros

Tony Bennett

Rickie Lee Jones

Van Morrison