Moses
Deuteronomy 27:9ESV·traditional attribution

Then Moses and the Levitical priests said to all Israel, “Keep silence and hear, O Israel: this day you have become the people of the LORD your God.

John Calvin Reformed @genevareformer

The ground of his exhortation is again taken from the special favor of adoption, wherewith the God of Abraham had honored them; for there was nothing which should have more effectually stimulated them to obedience, than that more than paternal love, and the gratuitous kindness with which He had prevented them.

Matthew Henry Presbyterian @wholebiblehenry

Here is, I. A general charge to the people to keep God's commandments; for in vain did they know them, unless they would do them. This is pressed upon them, 1. With all authority. Moses with the elders of Israel, the rulers of each tribe (Deu 27:1), and again, Moses and the priests the Levites (Deu 27:9); so that the charge is given by Moses...

Commenting on Deuteronomy 27:1-10

John Gill Reformed Baptist @doctorgill

And Moses and the priests the Levites spake unto all Israel,.... The priests who were Levites, as all the priests that were lawful ones were; and there were none but such at this time, who were. Eleazar and Ithamar, and their sons; these joined with Moses in the following exhortations to the people of Israel, as being particularly concerned in instructing them in the knowledge...