At number one I would have Michael Jackson and Barbra Striesand tied and ALONE, lol.
The rest, in no particular order
2. Steve Perry
3. Celine Dion
4. Stevie Wonder
5. Al Green
6. Donna Summer
7. Dionne Warwick
8. James Taylor
9. Sam Cooke
10. Patsy Cline
There's others that would be tide with some I put in the top ten, but just off the top of my head, those singers are in the top ten, in terms of pure vocal talent and then style. People like Billie Holiday and Frank Sinatra I would put in the top ten based purely on what brilliant stylists they were and for Holiday, emotive ability. Also people like Christopher Cross, Mel Torme, Karen Carpenter, all belong in the top 10.