Chemically speaking your breastmilk is tailor made for your baby.
Donated breast milk from another mother is next best, as it's still human milk. However, if you can't or won't breastfeed for whatever reason and donations are hard to come or make you nervous, it's a bit more trial and error, unfortunately. If the first formula you introduce works out, good for you good for baby. If it doesn't, it's best to talk to baby's pediatrician to see what to do next. Baby may have allergies or sensitivities, prone to gas or other complications that a doctor knows how to deal with quickly and efficiently.