What is best peel chemical for dark spot on face glycolic acid or tretinoin?

Chemical peel: There is no best. It depends on your skin color and the problem that you are trying to resolve . In general Alpha or beta hydro ya i'd peels are more effective than tretinoin.

Glycolic acid: Tretinoin is not a peel. It causes the cell to undergo more rapid mitosis which increases cellular turnover and hence an appearance of dry skin that in fact is not lack of moisture at all.