For me it's the Harry Potter series. I literally grew up with the characters. There's so much wisdom and depth in some of the quotes I find myself using some of those occasionally in day to day conversation with friends, and very often in my blog! To thus day I can just sit and binge watch hours of Harry potter theory videos on YouTube and then go back to the books and read again and validate the theories.
I've never read the Harry Potter books but have seen the films which I liked. I know what you mean about the depth of wisdom and thought provoking passages though - books have a way of speaking to us.