L'Oréal Paris Revitalift Filler Hyaluronic Acid Serum (£25) is so popular one is sold every minute around the world. According to the brand, its hyaluronic acid molecule retains a thousand times its weight in water, which, in turn helps to maintain moisture levels in the skin, keeping it feeling super hydrated. This serum (which you can read a full review of here) has been formulated with 1.5 percent concentration, which doesn't sound like a lot, but is actually the optimum amount to hydrate the skin without causing irritation.