Oh, don't get my started on Harry Potter.
I'm a big fan. It makes me so sad that the serie is now officially over; the waiting for a new book, the tension and the discussing of what would happen, and then finally getting to read it at midnight... I will never forget that, and it's sad it won't happen again. Perhaps there'll be other books like Beetle and the Bard, but it's not the same. I do hope there will appear an encyclopedia, though... that would be epic to say the least.
My favourite book is Deathly Hallows. Though I didn't really like the Hallows sub-plot (bit confusing, I prefered the Horcruxes) it was very well-written. Especially chapter 30, oh my. Best thing I've read in my life. And yes, I've read a lot. But how Rowling portrayed Harry's fear, his desire to keep living but nontheless giving in to the unbearable difficult taks to just willingly surrender... it gave me chills and tears.
My least favourite book is part one, just because it seems more childish than the other parts. CoS was definitely a lot scarier already. The books matured once the serie got older. You can't compare book one to book seven, though they're both full of magic, humour and adventure, and not to mention extremely detailed. I think that is the biggest strenght of the books: you can lose yourself in that world. Everthing, from sweets to newspapers to music to history is described and thought about. That makes it so very real and believable.
And the plot, of course. The plot. How can you not respect Rowling for that? That - just to name an example - Sirius Black is already mentioned once in the first chapter of the series, ulitmately becoming one of the most important characters? That already in book two something crucial to the plot appeared? (The diary). All this makes that you can read the books over and over without getting bored: you see things in a different light, you discover new things... and of course the always present humour (Ron, I love you for it) and descriptions. I don't know how many times I've read them, but it's certainly more than six times. And I owe the Harry Potter books in four different languages, having British and American verions... yes, I kinda like buying new ones. Though I have to stop because it costs a lot of money. Cough.
I also have to stop writing now because otherwise it will become a post TOO long! ;D (I'm incurable).