The youngest son
“But Yahweh said to Samuel, “Don’t look on his face, or on the height of his stature; because I have rejected him: for I see not as man sees; for man looks at the outward appearance, but Yahweh looks at the heart.””1 Samuel 16:7 · WEB
Israel asks for a king so they can be like the other nations. Samuel warns them exactly what a king will cost — your sons, your daughters, your fields, a tenth of everything — and they want one anyway. Saul looks the part: tall, impressive, and hollow. When he is rejected, God sends Samuel to Bethlehem to anoint a replacement from one family's sons. Seven walk past, each more plausible than the last, and God says no to every one: man looks at the outward appearance, but the LORD looks at the heart. The eighth is out with the sheep and has not even been called in. That is David. He goes back to the sheep afterwards. Being chosen by God rarely comes with an immediate change of circumstances.