This is a question that has been asked by many people and fans of the series. There are actually a few different answers to this question depending on who you ask.
Some people believe that Kirishima is Bakugou’s best friend since they have been friends for a long time. Some others might say that he is not necessarily Bakugou’s best friend, but he is one of his closest friends.
There is no clear answer to this question as it depends on what each individual person believes.