Depends on your taste but Id say on pure voice Sam Cooke..followed closely by Ray Charles and Aretha Franklin,,,,but again it depends on taste...Elvis and Michael Jackson definitely get a honorable mention as well.
one of the things i like about music is the variety of genres and singers with different quality of music and talent in singing. each voice has a different effect on our emotions. some singers have the gift of writing music & dancing while having substance in their music and those people have a true talent. for example, i like european symphonic and gothic metal so i would say 1. Anette Olzon of Nightwish 2. Tarja Turunen previously of Nightwish 3. Sharo den Adel of Within Temptation 4. Candace Night of Blackmore's Night 5. Chibi of The Birhtday Massacre 6. and the American we all know and love in the metallic rock world: Amy Lee of Evanescence