Within each pair of elements F and Cl, S and Se, and Li and Na respectively, the elements that release more energy upon an electron gain are

The first electron gain enthalpy is exothermic (or negative).
Generally, electron gain enthalpy becomes less exothermic (or less negative) when comparing elements of a group from top to bottom.

Therefore, electron gain enthalpy of S > Se and Li > Na.
But there are some exceptions to this.
One of them is the case of a group 17 elements where electron gain is most negative for Cl instead of F, due to extra small size of fluorine.
.'. Upon an electron gain, energy releases in the order Cl > F,S >Se and Li >Na.