V is the best member of BTS because he is versatile as an idol: he is not only a singer and dancer in the band but also an actor. In 2016, V joined the crew of Hwarang: The Beginning, a historical drama which aired for three months in South Korea. Since he is the only BTS member to successfully pursue an acting career, he has demonstrated his prowess and reached a level no other group member has been able to achieve.
Acting in a single TV show does not automatically make V the best member of BTS. He was not nominated for any awards for his performance, so his "success" in his role is determined only by what his fans say. Also, Jin went to college for acting and is formally trained, unlike V, so the former's acting skills may even be superior. The fact that V was in a TV show does not mean that he is the best member of BTS.