Ask yourself why one of your friends is not your best friends, may be your just extrovert and since you are always with people you may have strong relationship with many people and they consider you their best friend. Or you may you are introvert and your don't want to share intimate moments with a group, and your friends aren't those one to one kind. Or they just think you may not give the hype to closely have as best friend. I am uni student I still don't have "best friend" at least in defintion the one I can be myself in true and honest way. But my relationship with my some doormates is better than those I had in high school. So I think the exposure the amount of time you spend and mainly the things you share commonly to true with each other with friends matter to be best.