The books, without a shadow of a doubt. I was actually very disappointed with many aspects of the movies as they each came out. As a child, I was so excited to see my favorite stories depicted on the big screen, especially after watching the incredible adaption of Lord of the Rings into a movie that felt so true to the spirit of the books and yet adapted what was quite challenging to put onto the screen in such a masterful way. I know many fans started out with the movies and some even never even read the books, and that is totally fine. But the personalities of the characters are absolutely different and purposely made so in many complex and insidious ways in the movies with the way that the script and events were changed. There were things that neutered the characters personalities, odd changes such as significantly aging up Sirius and the other Marauders to middle age (50s instead of early 30s), which ultimately greatly changes their relationship to other adults in the story and to Harry and his friends. There are many more changes. Many people remark on how the script favored Hermione at the detriment of Ron, but Harry's character also greatly changed so that it was almost unrecognizable to me in the movies. His sassy and sarcastic nature, his anger and outbursts tempered by the love and loyalty he felt for those around him and his morals, and all that made him a unique and complex protagonist was very much watered down with a script and depiction that made him look like a calm, quiet, somewhat reserved normal boy who keeps being picked on by those around him and takes it in stride--the absolute opposite of his character in the books. Reminder that there is a reason the hat wavered between Slytherin and Griffindor when sorting him!