1. I was listening to the Temptations recently when I was overwhelmed by the fact that David Ruffin has undoubtedly the greatest voice in pop music history. I'll hear no dissension on the matter. His voice is a mixture of pure joy and pain in perfect equilibrium. These two elements were given a polished shine by the Temptations, the most precise vocal group ever. But don't take my word for it, Daryl Hall will tell you the same thing.
2. Dusty Springfield's voice had an ethereal quality. No vocal gymnastics and overdone melisma here - just a sound that is pure auditory ambrosia.
3. Most people probably know Sandy Denny from Led Zeppelin's haunting mandolin song "The Battle of Evermore", but anyone who's listened to Fairport Convention knows she possessed one the best voices you are likely to ever hear.
4. Not many would argue that Paul McCartney is among the greatest songwriters, but he isn't generally regarded as one of the greatest vocalists. Yet look at how perfectly his voice goes from the melodic ("All My Loving"), to the commanding("Lady Madonna"), to the bellicose ("Helter Skelter"); McCartney's voice always seems to fit perfectly. His voice harmonized brilliantly with Lennon's and could jump from soft to grandiose in an instant (i.e. "Maybe I'm Amazed"). Perhaps a guy like Clay Aiken (eegad!) can do more with his voice, but there's a lot to be said for just creating the right sound to fit the song.
And the rest...
5. Roberta Flack
6. Colin Blunstone (The Zombies)
7. Peter Cetera (Chicago)
8. Chuck Negron (Three Dog Night)
9. Toni Tennille
10. Tina Turner
Of course, you don't necessarily need great singing - guys like Joe Walsh may not be Pavarotti, but anyone else's vocals behind "Rocky Mountain Way" just wouldn't sound right. And it also depends on what mood your in - Barry White for that lovin' mood and Waylon Jennings for when it's time to drink yourself into a coma.
Worst voice? Robin Gibb sounds like a drowning chicken alone, but his voice harmonizes perfectly with Barry and Maurice. So, I guess he's okay.
Rolling Stone magazine's list of the top 100 vocalists can be found here. Although, I wasn't that impressed - it seems a bit predictable and off the mark.
BTW: The original golden throats, Frank Sinatra, Tony Bennett, Andy Williams, and Bing Crosby certainly sang with perfection, but they hit their peak before the 60s/70s.
And if you're wondering why octave showboaters like Jennifer Hudson and Mariah Carey aren't mentioned, you may want to read Reflections on Contemporary Music (and Why It Sucks) Part 2