Well, I believe more then one can be the best at the same time. We do have team sports. We have elite squads.
I just think it should be a choice to have more then one best friend, because everyone's social dynamic is unique. A person can start out with one childhood best friend. But, then grows up and meets their soul mate. Many people are best friends with their own parents or siblings. Sometimes it's a sibling and a friend from school.
I do use CC and I will look into the mod you're suggestion. Would it conflict with other mods?