Ascorbic acid—also known as L-ascorbic acid—has the most research of any form of vitamin C when it comes to benefits for skin. It is a potent antioxidant and skin-soothing agent that can improve the appearance of signs of aging.
Ascorbic acid is also a powerhouse when mixed with other antioxidants, or when used alone in higher concentrations, such as 15% or 20% or greater, amounts that can be great for treating an uneven skin tone.
A particularly vulnerable antioxidant when exposed to UV light and air, ascorbic acid must be packaged to protect it from these elements during routine use, or its effectiveness will gradually become diminished to the point of not working at all (you will see this as oxidized discoloration, meaning the product turns a copper to brownish color). For this reason, avoid any vitamin C (ascorbic acid) products packaged in jars.
References for this information:
Journal of Drugs in Dermatology, January 2012, issue 1, pages 51-56
AAPS PharmSciTech, September 2011, issue 3, pages 917-923
Journal of the American Academy of Dermatology, September 2008, issue 3, pages 418-425
Dermatologic Surgery, July 2005, issue 7, part 2, pages 814-817
Dermatologic Surgery, February 2001, issue 2, pages 137-142