Nah! Binitarian maybe ;-)
I'm going to plump for Mark. That's right, Mark: the Gospel we take refuge in because it doesn't rub that Trinity stuff in our faces. Yep, Mark has the most Trinitarian opening of them all:
"The beginning of the good news about Jesus Christ, the Son of God."
A whole theology is contained in the name "Jesus Christ, the Son of God". The true Joshua - the LORD who is salvation - comes among us. He is eternally anointed with the Holy Spirit (the Christ). He is eternally Son of the Father. And His coming is good news.
Why? Because, v2-3, He is the LORD of Isaiah 40, bringing "comfort" to the exiled people of God.
And He does it, v4-8, by joining us in our uncleanness and exile - entering into our baptismal waters, so that we might enter into His baptising Spirit. The Anointed One comes to draw us into His anointing.
As He joins us in our predicament, v9-11, the Father and Spirit do not abandon Him to our fate. The Spirit publicly refills Him, the Father publicly acknowledges Him. This is not only the triune identity declared in its clearest terms - it is the triune identity declared in salvation. The Son, as He enters into our exile, does so explicitly as the Father's Son, filled with the Spirit. The happy Trinity is passionately committed to our salvation: the Father sending His Son in the power of the Spirit.
And that Spirit, v12-13, drives Jesus into battle for us. Christ enters our wilderness and takes on our enemy as Champion - a true David to slay Goliath.
So here is the gospel, v14-15: the Kingdom has come because here is the King! Good news people, rethink everything, trust that God really has shown up to save, because here is His Spirit-filled Son!
From this point onwards Jesus engages every power that enslaves us: sin, sickness, Satan, a chaotic world, death. In every encounter with these forces, Jesus does not simply prove Himself superior. He proves Himself Saviour. All these powers dominate and destroy our lives. Jesus, the Spirit-filled Son, faces off against them in our name and on our behalf. If we belong to Him, His victory becomes our victory.
No wonder Mark opens by saying "Good News!" These are glad tidings of great joy. But only with trinity.
Without trinity, we simply have a Lord. And if we won't explicitly understand Him as Son of the Father, filled with the Spirit, we will seek to establish His identity in other terms. Without trinity, 'divine identity' become purely a matter of might. And, without trinity, the whole baptism thing will be a bit of a mystery. In fact we'll be hazy on most of the first 13 verses. We'll gain interest again right around verse 15: A call to repent! But since Jesus is introduced in vague terms as 'a Lord' we will construe that to mean "bow the knee"... or something.
And as Jesus takes on the forces of darkness in Mark's opening chapters we might consider these to be simply displays of power. We might just think that they establish "who's boss". And, again, the point will not be to reassure us that the Christ has entered the fray as our Champion, it will be to drive home the point that Jesus really is big. And we ought to... um... "bow the knee."
But with trinity we really will repent and believe. With trinity, we really will be overawed by our Jesus, the Christ, the Son of God. With trinity, the Gospel really is good news.