"Excellence by repetition becomes mediocrity. Cases in point. Joni Mitchell and Janis Joplin."

Sacrilege! Joni never repeated herself but stayed creative and iconoclastic throughout her career. Janis didn't live long enough to repeat herself.

If there is one female voice I would describe as "perfect", it's Karen Matheson's. There's a ton of her on Youtube, and most of it is hair-raisingly brilliant.