Chamber of Secrets could be my least favorite Harry Potter book.. I dunno why it just is. But this book has a great significance. It has the first horcrux, we are introduced for the first time to Dobby, and we get to know a little more about Voldemort, with a hint that Voldemort left a piece of his soul in Harry's, I mean I only saw this now, never noticed before. Some stuff didn't make sense to me (maybe cuz I am an adult now and not like the first time I read it when I was a kid). Still the book is magical. I specially loved the dueling chapter and as always Fred and George.. 
 Some quotes: 
 "When in doubt, go to the library." 
 "It is our choices, Harry, that show what we truly are, far more than our abilities."