Ever since I re-read the books as an adult I would say Dumbledore. We get bits and pieces where its serves the story and there is a bit more on pottermore but I think that once you read the book as an adult you realize that his story arc could be a stand alone series and he could be developed into a really interesting anti-hero if the book was for adults. I would love to understand more about his thoughts, his (at times) almost Machiavellian scheming as well as his development.