Tattoos placed over bones and tendons (spine, neck, back of ankle), on body parts with relatively little padding (feet, hands, joints), and anywhere with loads of nerve endings (nipples, fingers, face) will be the most painful.
My hips are good for grabbing when I'm bent over in my tiny black thong. I hope you bite my shoulder and neck while you're railing me from behind, handful of my hair in your hand, maybe a few (f)ingers in my mouth? (image)