That would be the wrong answer.
To say greatness is subjective is to claim that everyone’s answer is equally good, which would make a judgment about greatness meaningless.
Although preferences are personal, greatness is not. Greatness measures impact which is more or less an objective fact. I once made a group of students angry by claiming that Hitler was a great man, during a discussion of the “great man” version of history (where changes are driven by individuals, not social context). Hitler was great because he had impact, but was not good in any sense of the term.
Happily, with regard to wine, greatness does have to do with goodness. I can’t think of a wine that has had impact that was not also good in some sense. (Two Buck Chuck had impact. It is not a good wine but it was considered remarkable for the price and that value was its virtue)
A judgement about greatness in aesthetics refers to a level of consensus about quality as well as impact among those with the relevant experience. The Beatles are great because a consensus formed among a wide variety of intensely interested music lovers that their music is significant. Arthur Lee’s Love in the 60’s or My Bloody Valentine in the 80’s made good music but were not great because they lacked recognition.
That wines such as Penfolds Grange, Chateau Latour, Screaming Eagle, or La Tache are highly regarded is not a personal judgment but reflects a consensus regarding their importance. Consensus is one sign of objectivity although not an infallible criterion.
Of course, there are many ways of having impact, only some of which involve notoriety, and thus many ways of achieving greatness. An innovative wine might have a subtle form of impact on winemaking in a region even though it is not well known among most wine critics. Gravner’s skin contact whites are great because of their impact on that style of wine in Friuli, even though it’s hardly a household name outside the natural wine world.
Greatness is not necessarily noisy and it is not a zero sum game—that one wine is great does not make another wine less so.
But none of this has anything to do with subjectivity. We are all great in our own minds but true greatness is something else.