“What is the safest formula?”

You will have to determine which formula your baby is tolerating better. There are Cow milk formulas and Goat milk formulas that are gaining popularity. Start with one of those and see how much your baby drinks and moves bowels. Use that for at least two weeks, and if they are not drinking well, irritable, or moving their bowels, perhaps it's time to change, if they are doing excellent, stick to that brand until otherwise.

If you are in the US, ALL of the baby formulas have to pass FDA approval before they can be sold in the US so they are all safe for babies to consume.