In which compound is the hydrogen bonding the strongest in its liquid phase?

The hydrides of the first elements in groups $15-17$,namely $NH_3$,$H_2O$ and $HF$ respectively show abnormally high values for melting and boiling points. This is due to

Ethanol and dimethyl ether form a pair of functional isomers. The boiling point of ethanol is higher than that of dimethyl ether due to the presence of:

$KF$ combines with $HF$ to form $KHF_2$. Which of the following species are present in this compound?

What is the energy of a hydrogen bond?

Correct decreasing order of boiling point of given compounds:
$I. \ HCl$
$II. \ H_2O$
$III. \ HF$
$IV. \ NH_3$