I see what you mean. I just don't think it's as simple as choosing him as the greatest of his particular medium, and then choosing him as better in his medium than the greatest artists of other mediums are at theirs.
you might call him the "most influential", if you felt that way, for example. That's something quantifiable. But art is subjective, and what's the bestest ever to one person could be trash to another, in spite of the art's influence on viewers.