Which halide ion is the best leaving group when it is displaced from a primary alkyl halide $\mathrm{R}-\mathrm{X}$ in an ${\mathrm{S}}_{\mathrm{N}}2$ reaction?

Good leaving groups are weak bases, i.e. conjugate bases of strong acids. HI is the strongest acid among the hydrogen halides, so $\mathrm{I}{}^{-}$ is the weakest base and the best leaving group. The order is $\mathrm{I}{}^{-}>\mathrm{Br}{}^{-}>\mathrm{Cl}{}^{-}>\mathrm{F}{}^{-}$.