Well, from what I have learned in my Chem 11 class, like those above me have stated, electronegativity is the measure of an atom's affinity for acquiring electrons.
So by looking at the periodic table, you can conclude that Fluorine is the most electronegative element. Helium is a noble gas, which means that the orbital(s) is full. The s-orbital requires two electrons and it has just that. Since it is full, it doesn't have an attraction for other electrons of an element.
Generally speaking, you can use the periodic trends to help you with further questions like these.
We know that as the atomic number increases, the number of protons will too, so it will have a higher attraction for electrons, ie. a higher electronegativity. And as the atomic number increases down the group, we know that the atomic radii increases, because the electrons are at higher orbitals, making them farther away from the nucleus of the atom. The shielding in between the electrons reduce the attraction from the protons in the nucleus resulting in a weaker electronegativity. So from this, you can conclude that fluorine is the most electronegative element.
Hope this answers your question!