Anyone who tries to google skincare products hits a brick wall of fake reviews, SEO spam and hysterical pseudoscientific terror. Vogue's Christina Mueller writes that the blind-studied, peer-reviewed answer to your question is probably "Retinol":

Imagine for a moment that a revolutionary skin-care ingredient was discovered. It visibly smoothed out wrinkles and obliterated breakouts; it improved skin texture and tightened pores into tiny little nothings. ... Such an ingredient does exist, and chances are some form of it is currently languishing in a corner of your medicine cabinet. It’s retinol. It isn’t sexy. It definitely isn’t new. In fact, it was discovered 81 years ago, making it a veritable dowager compared with all the fresh new super-ingredients that have since come onto the anti-aging scene. For the past few decades, it has been hiding in plain sight—but with a few new developments, it is stepping back into the limelight.

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