Best is a noun when it is something. n. 1) One that surpasses all others. 2) The best part, moment, or value: The best is still to come. Let's get the best out of life. 3) The optimum condition or quality: look your best. She was at her best in the freestyle competition. 4) One's nicest or most formal
"We are the best of friends" = "We are extremely good friends."
"the best of [plural]" is an idiom of limited use; it's related to personal relationships. "the best of friends", "the best of buddies", "the best of neighbors".
There are other uses of "best" as a noun. Here's just one.
He did his best to finish the job. = He did the best he could to finish the