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.
I want to have Megan Fox on her knees with her hands tied behind her back and a ring gag holding her pretty plump lips apart while I thrust in and out of her warm, wet mouth
"very beautiful, little one; master is pleased. now bring your hand down your body slowly, then place your hands behind you on the floor and lean back, opening your knees wide as you do." (a girl poses for her master)