Using the stuff may sound like a no-brainer, but there's a caveat: Vitamin C products are notoriously difficult to concoct, as the ingredient loses stability and potency when in contact with air, light, and water — a little something that makes choosing a formulation seem impossible. So we conducted a little survey of our own, asking top derms and estheticians to share exactly which C serum they use in their own skin-care routines. Survey says: Our pollution-fighting and skin-brightening game is about to level up big time, thanks to their answers, ahead.