It's hard for me to choose, I think Jim Morrison had an amazing voice... but was more on the poetic side and never used his full music capabilities.
On the other hand Kurt Cobain balanced both out, while a good singer often times live he was very chaotic and couldn't keep the tune going (aside from selective performances such as the MTV Unplugged set) I think Joshua Miller has the right idea with Layne Staley.
Layne had such an amazing range, outside of his mantra echoing mic, the man was also pretty intricate both artistically and personally. I really think Layne was the all time best rock singer, and he did die quite young wasn't he 32? Anyways from songs like vibrating "Godsmack" to the cataclysmic "Them Bones" he could do no wrong with his pitch. His ability remained high, but his voice also took a toll due to his rather hidden but excessive lifestyle. Sadly I herd him do a rendition of "Another Brick In The Wall" for Mad Season and it was haunting to hear how soar his voice had become, much like Morrisons jaded vocals upon his final studio album with the Doors (L.A. Women).
While in his prime though I can't describe the qualities I felt from his abilities to connect with what he was applying himself to. He's a person I really I could have met. RIP Staley
Tracks I'd recommended (though many of you know Alice In Chains): "Down In A Hole", "Over Now", "Brother", "Love,Hate,Love", and lastly "Brush Away".