I saw Evanescence when they first came out, and while Amy Lee sounds good, she's a bit overrated.
I'll second Bjork, but it all depends on which song she's singing. She's got a few where I just wanna shove a sock in her mouth.
Jon Crosby (Vast)
Layne Staley (Alice in Chains)
Chris Cornell (Soundgarden, Temple of the Dog, Audioslave)
Eddie Vedder (Pearl Jam, Temple of the Dog)
Elizabeth Fraser (various Massive Attack tracks)
Siobhan de Mare (Mono, Violet Indiana)
"Only the educated are free" ~Epictetus
"Imagination is more important than knowledge..." ~Einstein