That's easy. Ray Robinson was along with Benny Leonard the best all around fighter in modern times. Why he even attracted an audience skipping rope...What a joy to watch him in his prime !
I think Leonard was more versatile than Hagler. Marvin could have trouble with slick boxers Leonard showed his versatility in the Hearns fight, when he became the aggressor and hunted Hearns down.
Given everything I've dead and the little footage I've seen, I think Benny Leonard fits this description to a T.