Which is more stable among the following ?

Larger the cation has more stable than smaller cation. The reason is if the cation to be small in size then it will undergo polarizability effect by the nucleus.

Hence, ${\mathrm{Li}}^{+}$gets polarized and it has been distorted of its shape.

But large cation ${\mathrm{Cs}}^{+}$does not undergo any distortion. Hence, ${\mathrm{Cs}}^{+}$is more stable than the other cations are given.