Due to small size atom show strongest back bonding in ${\mathrm{BF}}_3.$
In case of ${\mathrm{BF}}_3$ ($2\mathrm{P}-2\mathrm{P}$ back bonding) while in ${\mathrm{BCl}}_3(2\mathrm{P}-3\mathrm{P}),{\mathrm{BBr}}_3(2\mathrm{P}-4\mathrm{P})$ and ${\mathrm{BI}}_3(2\mathrm{P}-5\mathrm{P})$ back bonding. So extent of back bonding is maximum in $\left(2\mathrm{P}-2\mathrm{P}\right)$