I encounter this question often in my practice. Of course breast milk is best and by exclusively feeding breast milk through at least the first 6 months of age you reduce the incidence of allergies, asthma, and other chronic conditions. The length of time that a baby is breast fed is also directly related to a decrease chance of contracting meningitis. Solely breast feeding strongly boosts your baby's immune system as you can see.
However, if formula is necessary there is an amazing homemade formula you can find the recipe for in the book "Nourishing Traditions" by Sally Fallon & Mary Enig. It is by far the most balanced, healthy formula I have encountered in my practice. I have several families in my practice making it, and though it might seem overwhelming at first, people report that it takes just about 20 minutes every other day to make 2 days worth of formula.
After that, the next best would be a whole milk organic formula, if your baby can digest it.
The only other thing I'd like to mention is to never use soy formula. Soy formula in infants has been connected to thyroid disease as well as interfering with the metabolism of calcium. Actually we are one of the only western countries that still allows soy formula on the grocer's shelf.
Good luck & I hope this helps some. If you have any questions about what I have said feel free to contact me ###-###-#### or ____@____.com). Hope you're well.
J. Stevens, L.Ac.
www.yourwholefamilywellness.com