What is the best formula to use?

No difference. All commercially available formulas are relatively the same, and unless there is a special problem like lactose intolerance or allergy there is no real advantage of one over the others.
Cow milk formula. Start with a cow milk formula, unless there is a strong history of cow milk allergy in the family. The nutritional value of formulas in america are standardized, so they have similar amounts of calories, protein, carbohydrates, and fat. There are, however, different sources for these ingredients. For example, some formulas use cow milk protein, while others use soy protein.
Human milk. Baby seals get seal milk, baby elephants get elephant milk, baby humans get ? Cow's milk? Soy milk? The best choice going back to our origins is human milk, breast milk is best. If you haven't started formula -- don't! breast is best for baby and mom too.
Best formula. All formulas including store brand formulas contains the same nutrients your child needs. It depends on your child's particular need. For example if your child has milk protein allergy, reflux or is lactose intolerant, premature than he needs a special formula.
Inexpensive. Formulas are very strictly governed by the fda as to what they must, can, and cannot contain. So in the end, they are all very similar. There are minor differences that if a baby has special dietary needs, may be needed - you pediatrician will guide you with that. Otherwise, if you get some free, get a coupon for something, one is on sale - they are all fine to use and save yourself a few dollars.
Whatever works. Find the formula that nourishes your baby and allows them to feed well. The first weeks of feeding may be challenging regardless even if your choice of formula is the best for your baby. There are specific conditions for which certain formulas may be a better choice. Your pediatrician can help you should there be a problem.