Think of your best friend. Are you confident they also consider you their best friend? (I suspect this is quite common, but maybe not the largest group.)
Do you think there’s someone who considers you their best friend, but they are not yours?
Do you think your best friend has some other friend that’s their best? (This and the previous one are not mutually exclusive.)
Do you know you’re not anyone’s best friend? If so, are you fine with it, or does it bother you?