Commenting on this verse, James Edwards writes:
In the sublime declaration to Jesus at the baptism we encounter Fatherly love and filial obedience, kingship, and suffering service. Each is a facet of what it means to be the Son of God.James Edwards, The Gospel According To Mark (Pillar New Testament Commentary)
To no prophet had words been spoken such as the words to Jesus at the baptism.
Abraham was a friend of God (Isa 41:8)
Moses a servant of God (Deut 34:5)
Aaron a chosen one of God (Ps 105:26)
David a man after God's own heart (1 Sam 13:14)
and Paul an apostle (Rom 1:1)
Only Israel (Exod 4:23) and the king as Israel's leader (Ps 2:7) had been called God's Son before.
But where Israel failed [and all Isreal's kings failed!], Jesus takes [their] place.
Jesus is the new and better Israel! Jesus is the new and better King of Kings of Israel!