Which new born formula milk is best to feed concurrently w breast milk?

Nan is the closest to breastmilk but you still have to trial & error with the formula milks. Nan may or may not cause diarrhea which happened to my LO and the poop will be green, fart will smell bad. But I've also read that some babies had constipation with it. Afterall it depends on your baby's liking.