What is the most non-metallic element?

The most non-metallic element is krypton. Krypton is an odorless, colorless gas at standard temperature and pressure, and a cubic crystal when solid. Most metals are solid, bright, dense and silver or gray.

Krypton is effectively inert, because the outer shells of a krypton atom are completely filled. Many metals readily give up electrons to become cations. Moreover, groups of metal atoms have delocalized electrons that swim around their shared orbits and don't particularly belong to one atom or the other. This allows them to conduct electricity and heat.

Krypton also has no biological role, unlike metals such as copper or magnesium.