First of all, Im in love with these books since I was 12 years old, when I got my first book for Christmas. I was hooked then, and I am still now, several years, books, movies and theme parks later. One piece of me will always dream about this magical world, and Hogwarts will always be my home. 
 Having said that, this is one of my favorite books of the saga. Just second to the Order of the Phoenix. I just love how this book gets darker, and we get to know a little bit more about Harry's dad and that past. The story keeps getting better and better, and one can't help but be happy to enter that world once again and keep fighting the dark wizard! 
 Spoilers alert: One thing that will never make sense, is why they let Hermione use that thing to go back to the past to take some more classes, but not to save Harry's parents or something like that. I know it would hurt the plot a lot, but I think Rowling shouldn't have made something so powerful.