Glycolic acid and lactic acid are the most common AHAs found in skincare products, and have also been found to be the most effective in studies. Both are excellent at exfoliating the uppermost layers of skin to reveal rejuvenated skin underneath. Glycolic acid has the smallest molecule size of the AHAs, which means it is better able to penetrate the skin. Lactic acid has a slightly larger molecule size, so it works a bit more slowly. That being said, it may be more appropriate for those with more sensitive skin.