Cool post, Heather! This is an easy question for me. The book that has been a favorite for the longest is "Watership Down." I first discovered it when I was about 9 years old (before the movie), and for years growing up I read it at least once a year. It's been eons—literal decades—since I last read it and it probably wouldn't be the genre of book I'd tend toward today, but I buried myself into that one so many times and through so many dark periods that I know my answer without skipping a beat!