Among the haloacetic acids $\mathrm{XCH}{}_2\mathrm{COOH}$, which is the most acidic?

The -I effect follows the electronegativity of the halogen: $\mathrm{F}>\mathrm{Cl}>\mathrm{Br}>\mathrm{I}$. The more strongly the halogen withdraws electrons, the more stable the carboxylate, so $\mathrm{FCH}{}_2\mathrm{COOH}$ is most acidic.