Sam Cooke is my favorite, Whitney Houston is probably the best. It's not that hard. The best vocalist doesn't necessarily make the type of music I want to regularly listen to.
Perhaps it is more superficial than hard. Let me elaborate. I can tell you that Chris Cornell has a great voice. How do I know this? Because I know the general characteristics of what is considered a good voice, and when I listen to Chris Cornell sing, I can identify various characteristics that are consistent with those associated with a good voice.
Fact: I don't like Chris Cornell's voice. I never have.
So if I'm performing an exercise in identifying great voices, I am essentially just running a pattern recognition routine in my brain. I'm listening to music, thinking about what I just heard, and then forming a conclusion based on the analysis/thinking step. But that is completely missing the point. That isn't how music was intended to be consumed unless you're a music critic (and I hate music critics). Music should hit you from the very get go. You're either feeling it, or you're not feeling. The in-between analysis step is, again, something music critics do. I recognize that it's silly.
The lead singer of the yeah yeah yeah's is named Karen O. You may or may not have heard of her. She's a well known singer if you're into that type of music, and nobody would ever accuse of her sucking or being a hack, but I also haven't heard anybody rave on and on about her voice. And, again, I can tell you doing my "objective" analysis that she probably doesn't have what would be widely considered to be a great voice.
Fact: Karen O is my favorite singer. Her voice literally gives me chills...makes my hair stand on end.
So if someone were to ask me: "who is the better vocalist, Karen O or Chris Cornell?", how should I reply?
[youtube]http://www.youtube.com/watch?v=12TWXYoTQH8[/youtube]
[youtube]http://www.youtube.com/watch?v=oIIxlgcuQRU[/youtube]