Solubility of a salt depends on lattice enthalpy and hydration enthalpy.  Least soluble salt in water among the following is

${\mathrm{HCO}}_{3,}^{-} {\mathrm{Cl}}^{-},{\mathrm{SO}}_4^{-2} \mathrm{of} {\mathrm{Ca}}^{+2} \mathrm{and}{\mathrm{Mg}}^{+2}$   are soluble in water and are responsible for hardness of water;
${\mathrm{CaCO}}_3$ is insoluble in water.