I'm not sure...Judges, maybe? It honestly fluctuates, because I've learned things from most books of the Bible, so it depends upon what the book is teaching and what I'm going through.
But Judges, the "epic" book. The "exciting" book. The book with virtually no ritual Jewish stuff and no lists, where everything makes a lot more sense and you don't need a hefty commentary to just get through it. There are so few value judgements! It is terrifying to me, because they don't have the little clause "and the saying displeased the Lord..." They just give you the bare, brutal, ugly facts of life. Even the good guys are tied to the apron strings of a prophetess, have sons born of prostitutes, and use their God-given gifts to break their God-ordained vows. It is messed up and ugly; but even so, it serves to show us how merciful and forbearing God is.