James 5:16 says the prayer of what kind of person has great power?

Answer: A righteous man

"The prayer of a righteous man has great power to prevail" (KJV: "availeth much"); James then gives Elijah as an example.

“Therefore confess your sins to each other and pray for each other so that you may be healed. The prayer of a righteous man has great power to prevail.”— James 5:16 (BSB)