Which brother suggested selling Joseph to the Ishmaelites?

Answer: Judah

Judah asked, "What profit will we gain if we kill our brother?" and proposed selling Joseph instead.

“Then Judah said to his brothers, “What profit will we gain if we kill our brother and cover up his blood? Come, let us sell him to the Ishmaelites and not lay a hand on him; for he is our brother, our own flesh.” And they agreed.”— Genesis 37:26-27 (BSB)