Experts agree that breast milk is the best nutrition for a newborn baby. Breast milk is the best source of nutrition for babies during the first 6 months, which contains immunity-boosting antibodies and healthy enzymes that scientists have yet to replicate. There are also long-lasting benefits of breastfeeding for both the mother and the baby.
Are you meaning formula, breast milk, cow's milk, baby food? If so then it would be breast milk until 12 months. Then feed them formula until they are about 2, then it's just regular milk (at 4-6 months you can feed the baby, baby food.)