Prisoner of Azkaban. It was the first book I randomly picked up due to my peaked interest in the series, and I instantly fell in love and after I finished that read the books from the beginning and thus began my obsession.
Day 2 - Your favorite movie
Ironically enough, Prisoner of Azkaban as well.
Aside from it being an adaptation of my favorite book in the series, I felt like the film really captured the magical essence the book had. I understand that some people consider this their least favorite of the movies for whatever reason, but in my opinion Alfonso Cuaron had an innovative directorial style that was different from what Chris Columbus did in the first two films. It was a change from that same pattern we saw and I liked the creativity and amount of detail Cuaron added, to where it really seemed like a magical world you really wanted to be a part of. I think even JK Rowling at the time mentioned in an interview that there were hidden things for the future books that hadn't been released yet. I just absolutely adore it, and always will.
Rest of the 30 Day Harry Potter Challenge.
(I have been in a very Harry Potter mood lately, perhaps the sneaking realization that the final part of the final film will be released soon has finally gotten to me and I've become nostalgic and wanting to relive all the good memories I have had with this epic series so, forgive me if I start gushing over them over the next several days, or weeks, or months because dammit, I love my Harry Potter.)