Chlorine is the answer.
We now need to provide reasons. Consider electronegativity fluorine, which is the highest. Shouldn’t it also have the highest affinity? However, experimental results don’t support this claim.
Fluorine is small, so if it has an additional electron in its orbitals, the repulsions will be dominant. This an electron can be added to chlorine, but it does not face as many repelling electrons because it has an empty orbital. It also has a low electro negativity, so it has the highest affinity for chloride.
The affinites of the 3 period p block elements are higher than their 2 period counterparts. Ex Cl>F , S>O etc.