I would say excluding The French Opera, The Italian Opera, and NYC Broadway.
1. Little Richard
2. Elvis Presley
3. Freddie Mercury
4. Aretha Franklin
5. Robert Plant
6. Roy Orbison
7. Pat Benatar
8. John Lennon
9. Meatloaf
10. Sam Cooke
Source:
http://www.myelvisworld.com/triva/who-is-on-your-list-of-10-greatest-singers-ever/