The most common Best Friend Tag questions are simple questions about one friend's taste in movies or music, or something that someone who has spent enough time with them should know. If there is a funny punishment for getting the answer wrong, then the friends asks difficult questions to purposefully trip each other up.
The Best Friend Tag is a popular challenge on Youtube, where friends ask each other questions about themselves in order to see who is the better friend. Most of the time there are consequences for getting the answer wrong, and one of the most popular responses to a wrong answer is for the friend to throw a pie made of whipped cream in the other friend's face. The questions are mostly easy questions that a friend would know about another friend such as how they met, what their favorite color is, or what type of music they prefer, but if there are funny consequences then the questions become purposefully ridiculous.
The Best Friend Tag is currently one of the most popular tags on social media along with the Boyfriend Tag, which is the same things as the Best Friend Tag, just with relationship questions like how long they've been dating or where their first kiss was.