What is the most significant scene in the book?

This is more an opinion question. I don't think there is one right answer for this. I found the section where the great Oz is revealed to be poignant. This tells us that our fears are not always as scary as we make them out to be. The characters build up the Oz's power in their own minds while the reality of the situation, he turns out to be a small man, is quite the opposite.