Here's my answer:
Honestly if this isn't the most used answer for this question, I'd wonder what else it possibly could be. The fantastic thing about Harry Potter is simply how AMAZING the world building is, and how detailed J.K. Rowling's characters are. I wouldn't even need to follow Harry Potter, it could be any character, I just love the world and I have loved every character she has written. It is the reason why Harry Potter fanfiction is almost double the top fanfiction in any other category, and triple the next most popular book fanfiction (Twilight. Of course.) - and that's just looking at fanfiction.net, not all the other sites solely devoted to Harry Potter fanfiction. And of the Harry Potter fanfiction, they have actual categories specifically for Marauders as well as next generation, as well as all the characters we are familiar with, and there are loads of stories with a character the fanfiction author created and simply inserted into the Harry Potter world. And that's why I love these books so much. You will NEVER run out of things to do with these characters and their world. I could never get tired of reading about them (obviously, hence my large collection of HP ff I read haha).
I really loved the characters and the adventures that they went through in this series. I don't love the world nearly as much as I love J.K. Rowlings, but if C.S. Lewis had written more books for this, I definitely would have read them.
Oh my dear goodness I LOVE Tamora Pierce's books! I almost chose this instead of Harry Potter, that's how much I love them. Honestly any of her series would fit this. I am constantly choosing which series was my favourite (I haven't finished the Terrier or Circle of Magic ones though, so disregard them), although I'm most often switching between the Alanna and Trickster's Choice series. If I had to pick which one I want to have more of the most though, it would probably have to be The Protector of the Small quartet. I actually love that Kel is her own person and doesn't end up with a clear love interest. That being said...as much as I love it....I want to know!! I want her to be with someone! (I'm a Dom shipper, if you were wondering). I seriously reread all of these on a yearly basis. I get irrepressible cravings to go through their stories again. I wish I had more fanfic I enjoyed from the Tortall realm (so if you know of any, send me links. Please! I want them!)
And that's Day 1 of the 30 Day Book Challenge. What series would you have picked? If you're doing this challenge too, feel free to send me a link to your posts!