The grape scene. That's where it hits.
It was particularly hard for me because I'd read the entire series over the course of a couple of days, so everything was still fresh in my mind. You travel with the characters (and in this case a pivotal character), you become a part of their group, so when something happens you feel it's happened to a close friend. Vaughn did an excellent job with the character development and building this sense of vicarious osmosis. He's a truly phenomenal writer.