A Quick Tough-Grammar Tip:
What to Do When "Who" Could Be Plural or Singular

Which is correct?

"He's one of the authors who SAYS it best."

Or

"He's one of the authors who SAY it best."

Answer: "He's one of the authors who SAY it best."

If, on the other hand, you add "the only" before "one," the singular form of the verb is called for: "He's the only one of the authors who SAYS it best."

Just think in terms of the meaning being conveyed by the sentence. How many authors are we talking about? One person or more than one?