Which is the strongest acid among these: ##HBr##, ##HF##, ##HI##, and ##HCl##?

Acidity increases in the order: ##HF, HCl, HBr, HI##. Note that in water dissociation is fairly complete.
Acidity may be related to the reaction:
##HA rightleftharpoonsH^+ + A^-##
The stronger the acid; the further to the right lies this equilibrium. Of course, the acidity is modified by the identity of the . In water, the acid base reaction is often represented as:
##HX(g) + H_2O(l)rightleftharpoonsH_3O^+ + X^(-)##
In water, for all the hydrogen halides, EXCEPT for ##HF##, the equilibrium would lie almost quantitatively to the right. We could measure ##[H^+]## or its equivalent in another solvent, say acetic acid, and we would find the order of acidity as ##HI>HBr>HCl>HF##.
This clearly relates to the of the ##H-X## bond, and also to an effect. The larger halides, ##X^-##, are entropically favoured in that they are less charge dense, and cause less solvent order upon solvolysis.