What is the plural form of the sentence you are one of my best friends?

A sentence does not have a plural form. A sentence can be a compound sentence; two independent clauses, usually joined by a conjunction.

Your sentence has one plural noun, 'friends'; the pronoun 'you' can be singular or plural but we know that it is singular because it says 'you are one of'. The only way to make the subject pronoun 'you' into a plural is to drop the Words 'one of', making the sentence:

You are my friends.