From what I've read, the ribs are one of the most PAINFUL! The arms are said to be the LEAST painful. Good luck with whatever you choose! I know that my ankle hurt less than the lower back.
The majority of stories I've heard is that ribs hurt like hades. Many artists won't do ribs for someones first tattoo or they will give a major talking to over it. I've heard people describe it as excruiating and many never finish their pieces....I'm not trying to scare you..just trying to make you aware...