The strongest lewis acid among Boron Halides is ,

Generally , As the electronegativity increases , Acidic character increases but in case of Boron Halides, ${\text{BI}}_{\text{3}}$ is most Acidic, i.e, Strongest Lewis acid. This is because there is back donation of electrons from ‘F’ atom to Boron in case of ${\text{BF}}_{\text{3}}$ . This makes the ${\text{BF}}_{\text{3}}$ less electron deficient . This tendency of back donation decreases as the size of Halogen atom increases(due to large energy difference).
Hence,  is most electron deficient and thus the strongest Lewis acid.