Who knows about skin care more--doctors, cosmetics or drug companies?

Cosmetics. There is considerable knowledge among all the groups you mention. A dermatologist specializes in conditions of the skin, hair, and nails to best provide advice on the best products for your particular skin condition.
Dermatologists. A dermatologist spends 3 years in training about the skin, care of the skin, and treatment of abnormalities of the skin. While we need the products developed by the cosmetic and drug companies, the people in the companies have limited general information about the skin.