A reducing agent is a substance which can loose electron and hence a reducing agent should have low ionisation energy.

Now since ionisation energy decreases from Li to Cs, the reducing property should increase from Li to Cs.

The only exception to this is lithium.

This is because the net process of converting an atom to an ion takes place in 3 steps.

(i) M(s) → M(g) ΔH = Sublimation energy

(ii) M(g) → M⁺(g) + e^- ΔH = Ionisation energy

(iii) M⁺(g) + H2O → M⁺(aq) ΔH = Hydration energy

The large amount of energy liberated in hydration of Li(because of its small size) makes the overall ΔH negative.

This accounts for the higher oxidation potential of lithium i.e., its high reducing power.