There's a whole bunch of lists of "greatest ..." in the world of art. Does it mean influence, artistic excellence, ...?
edit: I've seen a number of greatest movies lists and I wonder what they mean if they don't specify what they mean by great.
If the makers of the list don't specify, it's safe to assume that the meaning of "greatest" is entirely subjective and that the only way to tell the criteria is from context--what made the list may give you a clue about what they were judging.
Otherwise, the only way to know is to ask the list curator/editor or better yet, to have them publish their criteria along with the list. Even then, when you're talking about art, the interpretation of whether a work best fits the criteria is almost entirely subjective.
Most publishers of lists like that are looking for attention to their subject matter (as RedGrittyBrick says), and are probably not keen to have the validity of their list picked apart. Keeping the criteria to themselves makes it harder to criticize their choices, so it's a popular action.
In this context, and many similar, it is a vague term used in headlines by greedy but weak editors either unable to think of an informative superlative or fearing that the following material is inadequate to justify one.
As always, "greatest" is a subjective term. When applying it, one must always ask, "Why?" Why is this great? Why is it significant? Why should it be considered better than other works? Why should it be set apart from the rest of art presumed, thus, to be less great?
It is all but impossible to separate mere opinion from objective judgment in such cases. Even the most educated judges are still human and harbor prejudices, likes and dislikes, and personal quirks which render their judgements open to challenge.
At best, any list of "Great Art" must be taken with a grain of salt. But that does not mean that one cannot learn from such lists. At the very least, when a work appears on many such lists, it lends some weight to the idea that the given work has objective merit. However, the very fact that many such lists exist and few works appear on all of them means that the body of possible "greatest" works is either very large, and all the works cited belong, or the body of possible "greatest works" is very small and only those included in the majority of such lists are truly "great." As in most such cases, the "truth" is likely to be found somewhere in the middle.
As a student of art, it behooves one to view as much art as possible, learn how art is created, learn the history of art, try to understand the movements, schools and trends in art through the ages and, ultimately form one's own opinion.
In the end, the only truly great art is the art that touches your spirit, that moves your heart, and engages your mind. Only that art is truly great... to you.