All of them!
Are you looking for something long-term or short-term? That can narrow it down to a shorter book (1 John, Ephesians, Colossians, Philippians, Esther, Ruth, etc.) vs. a longer book (John, Matthew, Romans, Proverbs, Isaiah, Hebrews).
it would be more of a long term thing.
we were considering job, ezekiel, john or acts