I don't blame you for being confused. As with any consumer product, each one advertises that it is the best or at least comparable to the best. Some say they have gentler proteins, some that they are better for development. How are you to know?
One of the factors in your choice should be your economic situation. Of course, breastfeeding is the most economical as well as healthful of all, but not everyone chooses to or can nurse. Women who are disadvantaged may qualify for the WIC (Women, Infants and Children) program, funded by the government to supply formula for those who need it. WIC usually has a contract with one of the major formula manufacturers. If you can afford it, the formulas that have the most DHA and ARA, fatty acids that support brain and eye health, may be slightly better according to current information. If cost is an issue, choose one of the less expensive brands, which still give your child good nutrition. Make sure the brand you choose is iron-fortified.
The major formulas are made from cow's milk. If your family has a history of allergy, especially severe food allergies, talk to your pediatrician about whether an alternative formula, such as a soy-based or one with smaller protein molecules, might be appropriate.
The only way you can know if a formula agrees with your baby is to try it. Be wary of switching formula every time your baby has a bout of gas. It is normal for infants to get periodic gas attacks in the first weeks of life. You may find yourself with the "formula of the week" if you chase those attacks with different formulas. However, your baby may need a special formula, recommended by your pediatrician, if she starts vomiting, has painful, hard stools, breaks out in an allergic rash, is not gaining weight well, or was born prematurely.