The Lewis acid nature of${\mathrm{BX}}_3$ follows the order :

The correct progression of these halides' Lewis-acid strengths is ${\mathrm{BF}}_3<{\mathrm{BCl}}_3<{\mathrm{BBr}}_3<{\mathrm{BI}}_3$. This can be explained by the halogen atom's propensity to donate its single electron pair to the vacant  through p-orbital of the B atom  $\mathrm{p\pi }-\mathrm{p\pi }$. bonding. Due to the equal size of the two p-orbitals in B and F, this tendency is stronger in F, which causes a decrease in B's electron shortage and the formation of ${\mathrm{BF}}_3$ acts like the weakest Lewis acid. The likelihood for reverse donation reduces as the size of the halogen atom increases from Cl to I, which causes the electron shortage of B to increase and the Lewis acid character to increase as a result.