Question

why does hydrofluoric acid among the hydrohalic acids behave as the weakest acid?

Answer 1

Among the halogens , Fluorine is very small and it has the highest electron affinity.  It attracts the Hydrogen ion very strongly. So  Fluorine does not sacrifice Hydrogen as much as  Chlorine or Bromine or Iodine.

So it is the weakest of all Hydrohalic acids.

Answer 2

This property of hydrohalic acids has nothing to do with Flourine's high electronegativity, that would if anything do the opposite and prove it to be a very strong acid. This is due to its very high bond dissociation enthalpy, meaning lots of energy is needed to break the H-F bond and release H+.