I'd have to say Hebrews. Love that book. So much good theology and some really strong warnings that kick me in the pants too.
I really like Psalms. The churches I've been in have not given nearly enough credit to it's artistic merit (imo).
Galatians. Really helped me when I was in a church context with a lot of legalism and lack of assurance
Genesis in the OT, but mostly because of the stuff after Noah. Romans in the NT, particularly chapters 7 and 8. Revelation is also up there too though.