Boyoboyo would I like to have the answer to this one!
Of course, no one singing style will please everyone...
but for starters, I imagine a great singer needs to have: sincere emotion, clear enunciation, true pitch, interesting repertoire, and elegant phrasing.
Singers we like to listen to, however, might have any combination of these traits. *Great* singers, I think, whether for opera, pop, rock, soul, or folk, need all of them.
fwiw, my $0.02!