[Anti-Aging] Which product has made the most significant change to your skin?

Regardless of price range, type of product (creams, serums, treatments, prescription, etc.):

I'm specifically looking for products available in the US for anti-aging, hyperpigmentation (Melasma), skin hydration, eye creams, help for fine lines, brightening, and help for a youthful appearance.

• What is your absolute favorite product/the product or treatment that has improved your skin the most?
• What changes has it made?
• How long did it take to improve your skin? How long does the treatment process take, typically (if applicable)?
• What is the price for this product/treatment?
• Do you lose the results if you discontinue use and/or when treatment ends (if applicable)?