El Elyon, The Most High God

He is the highest because He deserves to be. He is the only one who is good, so He is the only one who has the authority to rule.

And Melchizedek, the king of Salem and a priest of God Most High, brought Abram some bread and wine. Melchizedek blessed Abram with this blessing: “Blessed be Abram by God Most High, Creator of heaven and earth. And blessed be God Most High, who has defeated your enemies for you.” (Genesis 14:18-20)