Wich brand of infant formula is best?

Equal. No one brand is better than the other. If you still have the option, breast milk is the best nutrition you can provide for your baby.
None Proven. Every company touts their brand as best with specific ingredients that improve the babies health. Unfortunately there currently exists no long term data that any of these differences make a difference in the child's health as they get older. So it is entirely flexible at this time.
Basically the same? The fda regulates the composition of infant formulas, based on recommendations from various scientific entities. The brand of formula is not necessarily relevant. The type of formula, whether it is milk based, soy based, or semi-elemental, is of much greater importance. The protein and type of sugar or carbohydrate varies. The decision to change should be made with your pediatrician.
Baby's Only Organic. Breast milk is FAR better for your baby than ANY formula. Even a few weeks of breastfeeding is very helpful. But if breastfeeding is not possible then I strongly advise using organic formulas. A good article comparing available organic formulas is http://foodbabe.com/2013/05/28/how-to-find-the-safest-organic-infant-formula/ After reviewing the info in that article Baby's Own Organic seems best.
Baby decides. There is little difference between formulas. They have been regulated by the FDA since the early 80's and all have adequate nutrient blends.If formula is your choice, find one that baby seems to digest well and thrive, without GI symptoms ant stay with it.There is enough difference between formulas that a kid may have trouble switching back and forth.