In the end, this is really a question that you must answer yourself; it is a matter of personal opinion. To help guide you to the part that you found most touching, ask yourself a few questions. Was there any part of the book where you just had to keep reading, and couldn't put the book down because you needed to know what happened next? Or, was there any part of the book that made you feel a strong emotion--like sadness, or happiness? Did you laugh out loud or cry during any part of the book? Did any part make you angry or feel like you wanted to step in and make things go a different way? If you ask yourself these questions, it might lead you to an answer for yourself.
For me, the most unforgettable part of the book was near then end, when the bombs hit the city. The narrator, death, had been warning the entire time that Liesel's good friend Rudy would indeed die, but, I never really wanted it to happen. Rudy was such a great kid--I really, really liked him; I liked his honesty, his vulnerability, his love for Liesel, his patient teasing of her, the fact that he would go on adventures with her and ease her mind of its pain...all of these things really made me want Rudy to make it through and survive. But, indeed, Death did not lie, and Rudy is hit by the bombs after all. When Liesel leans over him and finally, too late, gives him that kiss that he wanted, that was an unforgettable moment for me.
Other great moments in the book to consider: When Liesel finds Max in the crowd of emaciated concentration camp prisoners, and steps out to speak to him--that is pretty unforgettable. Also, at the very end, when Max finds her again, and they collapse in relief and happy tears in each other's arms--that's a good moment. These moments, along with so many others, are what make this book so engaging and interesting. I hope that you can find one that moved you also, and relate it for your answer. Good luck!