Which halide of magnesium has the highest ionic character?

Size of anion is ${\text{I}}^{-}>\text{B}{\text{r}}^{-}>\text{C}{\text{l}}^{-}>{\text{F}}^{-}$. As the size increases, polarisation of an anion is more, hence when covalent character increases ionic character decreases.
Order of ionic character is $\text{M}\text{g}{\text{F}}_2>\text{M}\text{g}\text{C}{\text{l}}_2>\text{M}\text{g}\text{B}{\text{r}}_2>\text{M}\text{g}{\text{I}}_2$
Hence Magnesium fluoride has the highest ionic character.