The First chapter.
Chapter 1 is important because it introduces everything. It tells the reader the characters (or at least some of them), the setting, the main characters thoughts and emotions, and in doing so introduces the hero. It doesn't often tell the villain but shows the reader the life of the main character and the people around them. Does your place sound like a utopia? Do the characters think it is? If so, show (don't tell) how the utopia is really a dystopia in disguise, and whether or not the main character sees that. Does your main character think that the setting is a dystopia? Show (never tell) the reader why. When does your book take place? In the past? The present? The future? How is that world different (or the same) from today's world? Try to answer at least a few of these in your first chapter, but leave things unexplained, too. Some books leave so much unanswered that the reader may only keep reading to find out the answers, by which time they're hooked. Use questions to your advantage.