Zinc oxide and titanium dioxide are the only two sunscreen ingredients the FDA considers truly safe sunscreen ingredients, and as someone with a basic understanding of biology, I tend to agree with them on this (1). Zinc oxide and titanium dioxide nanoparticles refuse to penetrate the skin even when rubbed into open wounds and they have negligible effects in vivo when used topically. Additionally and crucially they block UVA, the major skin cancer promoter, which won't give you sunburns. Zinc oxide is the most effective broad spectrum sunscreen. Titanium dioxide is less effective with UVA absorption. All other sunscreen compounds function after being absorbed into the skin. Unfortunately they also get into the bloodstream where they act as hormones such as estrogen mimickers (unintentionally).

The only downside of zinc oxide and titanium dioxide is that they are awful to apply (you need a good technique) and you will probably look awful after application. It may be better to apply less material at a time but more often. Most manufacturers recommend every 80 minutes. I think this is defined by FDA regulation.

Since zinc oxide and titanium dioxide are semiconductors, they can both reflect and absorb UV. I think there is a large gap in understanding by sunscreen manufacturers of how these nanoparticles perform on a quantum level. I hold out hope that some chemist can further engineer these nanoparticles perhaps through an innovative coating to further improve performance and appearance when applied. I think there is still a lot of money being left on the table and these compounds are under appreciated, not understood well, nor developed enough to this day. Fortunately the sunscreens containing them do work today.

[1] https://www.fda.gov/media/153964/download