For me it's foundation and/or powder-foundation. They make the skin glow and hide imperfections. A girl can step out of the house and not worry about pimple scars, acne, dark circles, or lines showing when she's wearing foundation.
Vaseline aka petroleum jelly for me fits the bill for what is the most important cosmetic in my beauty arsenal. It does so many things that i actually just wrote a Hub about it. Armed with vaseline, you can moisten and bring out your lip color, use it on your eyelashes for a quick lift and then after the night is done you can also remove your makeup with it. You can also tame frizzy hair with petroleum jelly as it works to smooth hair while at the same time conditions any dryness. For you hands, vaseline is great for the cuticles as well as softens any rough spots on your hands and feet. I could go on and one, but for sure it is one beauty product I never leave home without!
For me, I'd say a good lipstick or lip color. It took me years to find a color that properly suited me, but once I did, it was well worth the search. Sometimes you might not need much other makeup. That and a good set of eyebrow tweezers!