Jimin is an extremely gifted dancer. The mere fact that he was chosen to be one of the group's main dancers is a testament to his ability. Jimin is one of the only BTS dancers with formal training; he studied modern dance in high school and came first in his class. Jimin is the best BTS boy because of his extraordinary talent.
Dancing talent is not the only factor that determines whether an idol is the best member of a K-Pop group. In addition, Jimin is also not the only BTS member known for his dance skills, and he has never won an award for his dancing, unlike J-Hope. It is debatable that Jimin is the best dancer, which then makes it doubtful that he is the best member of the group.