My brother said:
"The most exciting part of the book is when Jonas goes back to give the memories to everyone."
I would say:
Jonas, armed with memories of death and pain that nobody else has, knows exactly what's up. He watches in absolute horror as his father euthanizes a baby and this is something he knows he can never forgive or forget. This scene is the climax of the book because it sets in motion everything that comes after it.