Isn't your question much like asking what is a writer? Is it someone with a pencil or perhaps a pen or perhaps a typist? Do the words have to reach a certain level; is Shakespeare a scribe while I a mere scribbler? A long lost image of my grandparents would be a photograph for me, but perhaps only an old picture for you. In general, a good photographer can "capture" the image they intend to. I suppose any photograph could be rated on several categories and yet have some sort of "intangible" about it that sets it apart. Perhaps an image that causes more people than usual to pause longer than usual to view it or to come back to it to peruse it again, would be considered a "superior" image and the photographer who can produce such images on a somewhat regular basis a would be a "good", "better" (you pick the adjective) photographer. Your post opens a can of worms that will very hard to disentangle. There will be many opinions and most no more valid than others offered.