If I may ask, Rabbi, why is the question not well-defined?
After all, everyone understands what is meant when people say that Kevin Durant is better than Deni Avdija.

The question is not well-defined because it’s impossible to give a reasoned answer. When the difference is clear and in every respect, then there is an answer (and that is the case with Durant and Avdija). But when there are many different parameters, and in each of them one of the two has an advantage, and there is no way to weigh the importance of those parameters against each other, then the question is just pointless hair-splitting.
In this context I once mentioned something I saw from Rabbi Mazuz, who wrote that he has an article in which he proves that the Sha’agat Aryeh is the greatest of the later authorities. That gave me the same feeling.