ewg.org has a sunscreen guide http://www.ewg.org/2013sunscreen/9-surprising-facts-about-sunscreen/
The say that sunscreens are better at filtering UVB which causes sunburn which is a good thing, but don't filter out UVA very good which causes skin cancer and skin ageing.
Best is staying indoors, in the shade, or wearing long pants, sleeves, and hat with brim on all sides.
Zinc and Titanium oxides are good, but they stay on skin surface so come off easily, like there's this white stuff that gets on your clothes. They usually make it into nano-particles, so it's less visible, but then there are some questions about whether it will turn into dust, get into your lungs, and cause lung cancer
Avobenzone works some. It quickly breaks down so you have to re-apply. There's something (homosalate?, octocrylene, and octisalate) that makes it last a little longer before breaking down so you don't have to re-apply as often.
Oxybenzone, 4-MBC, and octinoxate may mimic hormones that interfere with normal development.
Vitamin A can cause skin cancer when applied to sun exposed to sunlight - exactly what you're trying to prevent.