I bet you thought sunscreen would be number one on this list. While sunscreen is pretty much the best thing you can put on your skin for protection, the number one spot goes to something far more important.
Sunscreen is the number one preventative topical against aging. It prevents more than fixes, but if you think about it, it's much easier to prevent the signs of aging than it is to reverse wrinkles, hyperpigmentation, or anything else that has already set in. Sunscreen protects you from the sun's harmful UV rays and prevents even more damage (and possibly cancer) from happening to your skin.