What is the most important part of John’s Revelation?”
I would contend the most important part of Revelation is chapters 2-3, the address to the seven congregations in Asia Minor.
Because like all books of the Bible, Revelation is specifically addressed to deal with behavior – the behavior of the children of God. That behavior is addressed comprehensively in chapters 2-3.
The Revelation (and that’s really the name of the book – not the “Revelations”) differs somewhat in the presentation of this material. In Paul’s letters for example, Paul usually tells why behavior must be a certain way, and then addresses the behavior itself. In Revelation, the why comes last.
The Christians at Sardis were depending on past faithfulness to win them God’s approval. The brethren at Philadelphia had gone through great persecution. Faith teetered on the brink. Jesus said: “hold on.”
The Christians at Laodicea found it difficult to decide whether they wanted to follow Jesus or follow the world.
All these are problems we face today. The Revelation points the way, and provides the motivation.