English is a very logical language.“Best” is an Adjective meaning “the topmost better than all others”.In other words, there is only one that is the “best”.Therefore, “only” is already implied in the definition of “best”, and saying it again in the sentence is redundant.This is how I would word the sentence:

1. He is my best friend.

2. He is my one and only friend (Note: That is, if you are a loner who doesn"t have any other friend).