I passed up a chance to see the Irish Rovers when I was 11 and still haven't forgiven myself.

Other than that:

Woody Guthrie
Hank Williams
Lefty Frizzell in his early years
the original Red Clay Ramblers
Bessie Smith
Ma Rainey
Robert Johnson
Norman Blake as he was in the Seventies
Buddy Holly
a young Bob Dylan
a young Arlo Guthrie (actually, I'd like to see the middle-aged Arlo, too)
Kate Wolf

and probably others . . .