That’s hard to answer because I like different books for different reasons. I love how Luke presents the story of Jesus. I’m drawn to 1 Peter because it speaks to us today as aliens living for God in a secular culture. I enjoy both parts of Daniel–the biography and the prophecy. And I love Judges–lots of action!
But I might say that Ecclesiastes is my favorite book. That may surprise some people because they have a negative impression. In some ways no book in the Bible speaks so clearly to the modern condition as Ecclesiastes. The writer asks questions you can find anywhere else in the Bible. And after surveying all that life has to offer, he concludes that we should enjoy life, remember our Creator, serve him while we can, and know that we won’t be here forever. I have sometimes said it is the “truest” book in the Bible, not that it is true and the other books aren’t, but it does speak to some tough areas of life that you won’t find anywhere else in the Bible.