UVC is what you want. It is not natural so nothing has any defense against it.
I deal with industrial UVC used for UV inks. We have some irradiators that pump out around 600 Watts. They are NASTY. If you expose one in a warehouse outside of the safety enclosure, it looks like someone opened the Arc of the Covenant in Raiders of the Lost Ark. It also creates an insane amount of Ozone.
The problem with these lights is they are very damaging to the human eye. You need to have safety goggles designed for these wavelengths. If you can look at it without goggles, it's not strong enough to be a germicide.
UVA and UVB also disinfect, but are not nearly as effective.