I've said this somewhere else on this Forum, but what the heck, I'll say it again!
My fave book is PoA, book #3. Of all the books, including #5, it breaks the formula . . . there is no face-down with Voldemort right before the end of school, just in time for spring break. You find out about Harry's parents' past and their friends; you learn about dementors, the Maruaders' map, and Hogsmeade; and as Stardust said, reading it the first time is so suspenseful, and then on re-reads, you learn things you didn't know before! Also, Lupin is one of my favorite characters . . . I love his relationship with Harry and how he teaches the DADA class. I really enjoyed book 5, and will, no doubt, read it over again several times, but book 3 edges it out and remains my fave.