${\mathrm{BX}}_3+{\mathrm{NH}}_3\stackrel{\mathrm{R}\cdot \mathrm{T}.}{⟶}{\mathrm{BX}}_3\cdot {\mathrm{NH}}_3$ + Heat of adduct formation($∆$H) The numerical value of $∆$H is found to be maximum for :

There is effective overlapping between B and F in BF₃, therefore it is the weakest Lewis acid whereas ineffective overlapping between B and I in BI₃, therefore it is the strongest Lewis acid.

Lewis acidic strength: ${\mathrm{BF}}_3<{\mathrm{BCl}}_3<{\mathrm{BBr}}_3<{\mathrm{BI}}_3$

Stronger is the Lewis acid, more heat is released during the reaction.
As BI₃ is the strongest Lewis acid among all boron halides therefore, the heat of adduct formation will the numerically maximum for BI₃.