personally same thing guides me as i posted my favorite guitarists on another thread...uniquness and style not technical proficiency...who had brought a new voicing, an new style, differnt phrasing etc? who has soul?
in no particular order, the ones who come to mind are
frank sinatra, robert plant, smokey robinson, james brown, john lennon, louis armstrong, muddy waters, marvin gaye, ray charles, ...
many of the sighted in this thread (certainly not all! and just because I have not included my list doesn't mean so...just for sake of a degree of brevity)...I find more iterations of the true pioneers, individualists. for example, i really like what's his name from Incubus: but would I put him as one of the great ones: no, he is really good, but is not someone who has really brought something unique. someone who may be getting there among current artists, I think is seal