"Greatness means different things to different people."
That's right, unless an effort is made to identify criteria that characterize "greatness." This isn't a hopeless task, but it is a laborious one.
Wouldn't we agree, after all, that "greatness" implies qualities such as the following?
--If a work is great, it will show the simultaneous and interpenetrating achievement of things that belong together. For example, descriptive passages do not seem to be imposed upon the story, but rather they are necessary for the story to be told well. The Lord of the Rings excels in this, as does Titus Groan.
--Perhaps an aspect of the previous point: the diction (word choice) is right for the work; one never feels that the work is a "good idea" but the "style" is defective. Impatience with the style of the work shows defect in the reader, not the work. This, I believe, is true of The Lord of the Rings, which by the way possesses not one style but a variety of styles appropriate to character, occasion, tempo of the storytelling, appropriate atmosphere for an episode, and so on.
--The work possesses that degree of characterization that is appropriate to it. ("Characterization" shouldn't be discussed apart from this. The degree of "characterization" appropriate to one story may be quite different from what's appropriate to another.)
--The work isn't made with an inferior alloy derived, for example, from contemporary trends, obsessions, and notions. We feel that the artistic integrity of an imaginative work is compromised when it betrays an undue attention to such things. A great work may, however, be intimately related to identifiable current events. The example that comes to my mind is Dostoevsky's Demons, which isn't fantasy. It is a tremendous work the imagination, though.
--The work should possess wisdom. It is not simply clever. This may be one of the most difficult criteria because our culture seems to have little sense of the difference. An indication of this: the academic world is swamped by clever professors, lectures, articles, conferences, symposia, position statements, podcasts, whatever, but (I think) precious little wisdom. Cleverness often pivots around wishes about the way things should be. Wisdom deals with real things, including the perennial truths. We live in a post-wisdom culture. I'd better not say much more about that here.
--If the literary work is fantasy, it must also convey or work "enchantment." I keep adding to this posting already so I won't here go into what I think makes for literary enchantment, but I would resist the too-easy idea that, shucks, it's just a matter of taste.
If we work at it, we can achieve quite a bit of consensus about what makes for literary greatness, I suspect, but that's not a discussion for the fainthearted. But when a fair degree of consensus is achieved, then we can have better discussions about the works that deserve to be called great -- or so I suspect. I do think we can also work inductively (if that's the right word). We can take a work that most people agree is "great" -- The Lord of the Rings -- and try to put into words the qualities it possesses so superlatively.