Before I talk about the books and my relationship with them to try and explain why they wound up on the list a few words about the above covers. I personally prefer the original covers that adorned my post about the series in the earlier list, but I thought I'd put these there for some variation. Yes, this is a series and not a single book. Trying to pick just one book out as the favourite of the series is like trying to pick out your favourite chapter from a loved book. While they are relatively self contained in that each book covers a school year of Harry's life, it's still one continuing story told in 7 books.
I wasn't an early adopter with Harry Potter. I had seen the first book a few times, picked it up and had a bit of a flick through, but it always struck me as too young for me (I was an adult when they came out), then Goblet of Fire was published and caused a big enough stir that stories were shown on the news about people camping out to be first in line to buy a copy and maybe even catch a glimpse of the author. I began to reassess my earlier stance and once the first film hit the box office and went bananas I decided to see what all the fuss was about.
Despite my earlier feelings about how 'childish' the first book was, I found myself drawn in by it. I think by the time I hit the Mirror of Erised I was hooked and in for the long haul. What J. K. Rowling did that I couldn't remember many other similar authors doing (she didn't create the idea of child wizards or magical schools) was let her title character grow up physically, as well as mentally and emotionally, with each successive book. Even if the characters in similar series age physically and chronologically there's generally not much growth mentally or emotionally. What Rowling set out to do was have Harry and his friends age with the target readership. Philosopher's Stone came out in 1997 and The Deathly Hallows in 2007, so 10 years overall, which was only a few more than the years Harry actually spent at Hogwarts. So a reader could have conceivably started reading the books at the age of 10, and have turned 20 by the time the final one came out and therefore grown up with Harry.
For it's failings, and they are few despite what the many nay sayers want to claim, the Harry Potter series introduced a generation to the joys of reading and to magic in books. If a reader also wants to scratch the surface they'll find some very clever writing tricks and deeper meaning behind the words on the page.