Vitamin A aka Retinol

Vitamin A chemical name is retinol, the alcohol form of vitamin A as it occurs in nature.
In the skin retinol is first oxidized into retinal then retinoic acid.

Because retinol is very unstable, esters such as retinyl palmitate are used but you add an extra step into the transformation and they are less effective.

I favor encapsulated retinol, retinal or the cosmetic base made of PEG, PEG 4000 to enhance the penetration of retinol thus increasing its activity.