In good 'knb x reader' fanfiction, the relationship between the reader - insert and the KNB characters should be believable. It shouldn't feel forced. Whether it's a friendship or a romantic relationship, there should be a natural progression. For instance, if it's a romantic relationship with Aomine, there should be moments of conflict, understanding, and growth between the two characters. The story might also include details about the KNB universe, like the rules of the basketball games and the rivalries between the teams, which adds depth to the overall story.
A great 'knb x reader' fanfic usually has an engaging plot. It could be something like the reader - insert joining the basketball team as a manager and getting involved in the team's drama, or it could be a story where the reader has some special ability that interacts with the KNB world in an interesting way. The plot should keep you hooked from start to finish. And of course, the writing style should be smooth and easy to read, with good grammar and proper pacing.