Greatest singers

Now we know some of your favorite guitar players.
My question who is the greatest singer of all time?This is also a hard one to list only one person.Who are some that should be in the top ten?
I think Elvis and Roy Orbison should be among the top ten.There are so many.And I have not even gave Country music any thought.
Some of may favorites Paul Rogers of Bad Co.,Freddie Mercury ,Robert Plant ,Bon Scott of AC/DC,Bob Segar,Steve Perry,Dennis DeYoung of Styx,Elvis,and Pat Benatar.
Jim