Whatever works. The basic content of the formulas has been regulated since the early 80's.Each contains enough of needed nutrients for most babies to thrive, but they are not interchangeable.There are 3 basic types,cow milk based,soy based and specialty.Some babies may not tolerate one or more types,but your doc can help you find one if that happens.Find one that works and stay with it,switching can cause problms.
See below. There are multiple formula preparations available. They are manufactured to the same standards. Formula manufactures want to make their formula as close to breast milk as they can. Some formulas have added nutrients and supplements that may or may not be beneficial to your child. But in general, they are all good. Save yourself some money and by non brand name.