Among ${\mathrm{MgCl}}_2,\mathrm{RbCl},{\mathrm{BeCl}}_2$ and $\mathrm{LiCl}$ the compounds with the highest and the lowest % of ionic characters are :

The size of the cation grows with its ionic character, hence among the listed chlorides, Rb has a greater sine, and Be has the smallest size. $\mathrm{RbCl}$ is the compound that has the most ionic property.
${\mathrm{BeCl}}_2$ is the compound with the most ionic characteristics.