Higher the lattice energy lower the solubility. Out of the four combinations possible, the lattice energy of MgS (bi-bivalent ionic solid) is higher than those of $\mathrm{N}{\mathrm{a}}_2\mathrm{S},\mathrm{ }\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2$ (uni-bivalent or biuni-valent ionic solids ) and NaCl (univalent ionic solids) and hence, MgS is the least soluble. Thus, due to higher charge/ion, the lattice formed between Mg²⁺ and S^2- is very strong.
As lattice energy overweighs the hydration energy here, the salt is least soluble.