The most widely used Alpha Hydroxy Acids are Glycolic Acid, Lactic Acid, Citric Acid and Malic Acid.
Of these, Glycolic Acid has the smallest molecules and so can penetrate your skin more deeply. Making it the most powerful exfoliant of the AHAs.
However this can come at a cost - leaving some types of skin irritated and red.
Lactic acid is a larger molecule and is a much better choice. It can gently exfoliate without irritating your skin.