Which is the best formula to feed my baby?

Cow milk formula. 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 the ingredients. For example, some formulas use cow milk protein, while others use soy protein. The type of sugars and fats in the formula may be different also. Infant formulas generally have 20 calories per ounce.
Coke vs pepsi. Without any irritability, enfamil and similac are like coke and pepsi. Take your pick. If baby irritable should get evaluated for milk protein allergy, which might need nutramigen or alimentum. If no allergy, i find gerber goodstart is a good choice.