Teachers ask this question with the hopes that you will elaborate on the part that you pick. Think about what the teacher has been focusing on in class and tell him/her about that along with your favorite part. For example, if you have been learning about the elements of a novel (i.e., exposition, plot, characters, setting, climax and resolution) you could explain where your favorite part lands in that structure. Personally, I like John's personality in the book and it comes out immediately at the beginning of chapter 1 when it is his turn to speak in the story. The most interesting is when he introduces his nickname as the Bathroom Bomber; right then the reader knows that this guy is a fun and interesting character to follow (1).