As usual, people are offering their personal taste, as against answering the question.
In essence, it is possible for a musician or band to be very highly rated, as in critically acclaimed, rather than commercially successful, but not actually be worthy of that acclaim.
So using those parameters, I would have to cite Frank Sinatra, because I believe his famous 'phrasing', and very few people actually know what that means, is not really all that, and I find a singer like Matt Munro to be technically more skilled as a vocalist.
I have no personal axe to grind, I am not a fan of either singer or the genre, so I can be objective about acclaim based against actual ability, and in that, I find Sinatra seriously wanting.