The instant messaging service "Snapchat chat" is a new feature on the snapchat app, but it does not affect who is shown as your best friend.
Your best friends on snapchat are the contacts that you send snaps to most frequently, or who sends snaps to you. A 'snap' counts as a picture or video that you send someone, not you sending them an instant message.
There is a simple explanation to this, as snapchat best friends are calculated depending on your 'score' with that person.
Seeing as Snacpchat chat does not affect score, then it make sense for it not to affect best friends either.
This means that if someone sends you snaps a lot, and you don't send them any, then they could still be shown to be on your best friends list because the score between you will be high. The same applies to if you send someone a snapchat a lot, but they don't send you any, they could still be a best friend because of the score that you have accumulated.
A 'score' is basically the amount of snaps that you have sent to someone.
It is impossible to choose who your best friends are because the app always uses this system to work out who your best friends are. This can sometimes lead to awkward situations, as your best friends are available for everyone to see!