What formula is best for a premature baby?

None. Breastmilk is best for a premature baby. Unfortunately, formulas are a poor substitute. They are harder for babies to digest, especially for premature babies, and they don't contain all the nutrients that breastmilk does. If for some reason you can't provide breastmilk, the baby's doctor can recommend a formula. Sometimes they end up trying several to find one that a baby can tolerate.