The short answer, of course, is, “Anybody He wants.” But if we think the question through more thoroughly, what we’re probably looking for is some characteristic in or about the person God uses that makes him or her most useful.
Two areas we often look, which are no special indicator of usefulness, are qualifications and giftedness. Qualifications are often more about rights than responsibilities. “I’m qualified in this or that way; therefore, I should be allowed to do or not do _____ .” Giftedness, on the other hand is more about potential than reality. The pathways of life are littered with gifted failures.
The best general indicator of usefulness might just be obedience, and a good example of that is found in the prophet Amos. The Scriptures give us no indication that Amos was a gifted speaker. Nor did he come into his ministry with any special education or experience. Nonetheless, Amos was willing and would do what the Lord called him to do.
The Lord called Amos to preach to the people of Israel, including Jeroboam the king and Amaziah the priest of Bethel. So Amos spoke and Amaziah told him to be quiet. (In that setting Amaziah was the one “qualified” to speak.)
“Then Amos answered, and said to Amaziah: ‘I was no prophet, nor was I a son of a prophet, but I was a sheepbreeder and a tender of sycamore fruit. Then the Lord took me as I followed the flock, and the Lord said to me, “Go, prophesy to My people Israel.”’” – Amos 7:14-15 (NKJV)
So God spoke through Amos, the shepherd and fruit-picker-turned-prophet – not because he was a shepherd or a fruit picker, but because he was obedient. Amos went and did what God wanted – and no doubt He wants us to go somewhere, do something or say something too.