S-block elements are strong electropositive elements with low reduction potential, indicating their strong reducing ability compared to others. So, substances having lower reducing ability than them will not be able to reduce them. Reducing the ability of an atom is related to the ease of releasing electrons for reduction. Decreasing ionization energy down the column suggests that caesium is a stronger reducing agent than Lithium.
But, reducing ability (oxidation potential) depends on the combined energy difference of three processes:
Sublimation of the atom
Ionization of the metal ion
Hydration of the ion with water
Lithium, being the smallest ion, its hydration enthalpy is very high than caesium and compensates more than its higher ionization enthalpy. Thus, lithium has the highest reducing ability (highest oxidation potential or lowest reduction potential = -3.04V) compared to caesium.