Any of them? I personally am refraining from much of the New Testament because so much of what Christ says and does resonates to me on a completely different personal level than I find a comparison with OT books. That's not to say I don't hold the OT higher, but I think it's unfair to compare the wisdom of old to the wisdom of God. Correct me if I am wrong.
I've been reading through the Book of Job again and it brings to my attention just how wonderful this book is. Even on it's own, it would easily rate as one of the greatest books in all of human history, it's an all time classic. It's haunting, beautiful, mysterious, thought-provoking, etc. What it means to me puts theodicy in an entirely different perspective. Instead of questioning God for suffering, I instead turn the question back onto myself; God acts like the great Questioner, reversing the roles and we answer the questions He gives us. It does put my mind at rest in regards to human suffering, maybe not so much child suffering, but on an adult level it does. I find solace in knowing God's Will is itself a mystery.
Another book I find just as fascinating is Ecclesiastes. I believe this is the only book in the Bible where God does not speak? But this philosopher is in the vain pursuit of meaning and purpose. Here is where any God-substitutes merely fail; the emptiness of removing God to a position of remoteness and irrelevance. This very emptiness that modern man tries to cover up with thousands of distractions, and Ecclesiastes blows it the cover.
I am very thankful that the Jews included these two books in their canon.
I guess a third book that always resonated with me was Revelation, and this was the only book of the Bible I ever read as a child. It fascinated me with its imagery and symbolism that would allow my imagination to run free, to thinking of the vision of the New Jerusalem in all its glory. It doesn't resonate to me that much anymore as it did as a child, but I do turn to it to remind myself of the awesome redemptive power of God and his love and mercy for His people.
Proverbs and Sirach I also highly enjoy for its wisdom and guidance.
I don't think there is a bad book in the Bible, but there are few the resonate with me more.