All time is too hard.
I would say Pavarotti but i have come to not respecting him as a person.
Korean is obviously Sohyang
Bocelli, another of three tenor's in classical world
Elvis, Ray Charles, Frank , Bing, Louis each have a niche.
Sarah Brightman
Aretha Franklin
then there are so many whom we don't have a super quality audio to judge Caruso, Fitzgerald ect.
Female: these 2
lets say Aretha; Fantastic for decade's, took care of her instrument, multiple genres.
Sarah is not a bad call originated many of the biggest stage plays of all time and does pop and classical. 100's of world wide hits.
Male its harder. ill have to think