personally, I'd say calcium.
but here are some ways that theses things help the body. Maybe they might help you decide.
PROTEIN: Hair and nails are mostly made of protein. Your body uses protein to build and repair tissues. You also use protein to make enzymes, hormones, and other body chemicals. Protein is an important building block of bones, muscles, cartilage, skin, and blood.
CALCIUM: Calcium is a mineral that is necessary for life. In addition to building bones and keeping them healthy, calcium helps our blood clot, nerves send messages and muscles contract. About 99 percent of the calcium in our bodies is in our bones and teeth.
FIBER: Soluble fiber dissolves in water and becomes a gelatinous form, which may help to slow digestion and help your body absorb vital nutrients from some foods. Insoluble fiber stays in its fibrous form, helps food pass through the digestive system and adds bulk to the stool, which helps you to stay regular.
