Keep it simple. First, seriously consider nursing. If not possible, and your baby has no allergies or problems, use a milk based formula. If baby has health problems ask your doc for help. There are lots of gimmicks from formula companies right now touting their formula as best. In reality, formula is very closely regulated by the fda and not much difference between the brands unless your infant has special needs.
Most appropriate. You want to pick a formula that is going to be easy to digest and affordable. Powdered formula is cheaper than liquid formula. You may switch to a soy based formula if your baby is sensitive to cow's milk. You can also try a thickened formula if your baby spits up a lot. Iron-fortified formula is not necessary until 4 months of age and can lead to constipation.