Which is the most electronegative element

Fluorine.

Explanation:

The most electronegative element is fluorine. Electronegativity is the ability of an single atom to attract electrons, mainly in covalent bonds. So, fluorine is the element that can attract most shared electrons in any interaction involves through this type of bond (covalent bond).

This phenomenon develop strong structures, for example, we can find fluorine in bones formations. Because of its high electronegativity the excess of this element in our body could bring negative consequences in the nervous system.