$BF_3$ is a weaker Lewis acid than other boron halides due to strong p$\pi$ - p$\pi$ back bonding in $BF_3$
In $BF_3$ each fluorine has completely unutilised 2p while boron has a vacant 2p orbital. Now since, both of these orbitals belong to same energy level (2p), they can overlap effectively as a result of which fluorine electrons are transferred into the vacant 2p orbital of boron resulting in the formation of an additional p$\pi$-p$\pi$ bond. As a result of back donation of electrons, the electron deficiency of boron reduced and hence, Lewis acidity is decreased.