Among the halogens, which element has the most negative electron gain enthalpy?

The electron gain enthalpy of Cl ($-349\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$) is more negative than that of F ($-328\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$). The 2p subshell of F is so compact that the added electron suffers strong inter-electronic repulsion. Br and I are less negative because of their larger size. Hence Cl is correct.