Psalm 133 compares brothers living in harmony to oil running down whose beard?

Answer: Aaron's

Unity is "like fine oil on the head, running down on the beard, running down Aaron's beard over the collar of his robes."

“A song of ascents. Of David. Behold, how good and pleasant it is when brothers live together in harmony! It is like fine oil on the head, running down on the beard, running down Aaron’s beard over the collar of his robes.”— Psalm 133:1-2 (BSB)