the prisoner of azkaban was definitely my favorite when i was younger. [insert old person's voice] i think i just loved that more of the world around magical "beasts" was explored and the introduction of animagi. today, maybe it would be the first book because i love how it starts off with the thorough (and very funny) description of the dursleys and harrys life there and then - bam - you're a wizard harry and then all the cool stuff happens you know? like magic and such.