... it sounds like you're at primary school.
I know a few people who refer to their 'best friend'. Maybe it's just me, but I can't help but think it makes them sound like a child.
Why not just call them a close friend? What happens if they start to drift apart from their best friend, at what point do they suddenly stop being the 'best friend' and do they need to bagsy another one quick?
Disclaimer: I only have a few real friends, some are closer than others, and none really qualify for a 'best friend' so I may well be envious!