For me the only two reasons for not getting a tatto are:¨
1. The Pain.
2. The Permanentness. I Change my desktop picture at least once a month, if not more frequently. I can never say what my favorite movie/song/book/character, because i'm always changing my mind. I predict getting a tattoo for life would be something i would be ok with for a maximum of ten years, i would definitely change my mind about what i wanted to be permanently on my flesh in that time.
But since you say i "have to", i have always considered that i i got one, it would be a Transmutation Circle from Fullmetal Alchemist. I find them intriguing and geometrically beautiful (perfect circles, perfect squares, perfect symmetry, etc.)
I would Probably go with the human transmutation circle (the first image), since it's the most iconic, across my back. Though a Simpler version on my arms or hands (for easy use XD!), sound funs, and will probably involve a lot less pain.