Snapchat simply ranks best friends on the amount of Snaps sent and received by that user.
So to get someone to show as your best friend, all you need to do is send each other as many snaps as possible.
When you are the highest among the people you communicate with, you will become Snapchat "best friends"
But if you aren't worried about who Snapchat says is your best friend, then just keep using the app how and with who you want!
What does it matter that you send more snaps to one person than the other? Does that really make you best friends? Or is it what you guys do IRL (in real life) that counts?
So there's no mystery to how Snapchat best friends are ranked, but my suggestions is just don't worry about it!