The strongest lewis acid among Boron Halides is ,

Generally , As the electronegativity increases , Acidic character increases but in case of Boron Halides, BI3 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 BF3 . This makes the BF3 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.