The individual dipole moments in ammonia (NH3) do not cancel each other

out. What is the strongest force that exists between molecules of ammonia?

The strongest force that exists between molecules of Ammonia is Hydrogen Bonding.

Explanation:

Hydrogen Bond Interactions are those interactions which are formed between a partial positive hydrogen atom bonded directly to most electronegative atoms (i. e. F, O and N) of one molecule interacts with the partial negative most electronegative atom of another molecule.

Hence, in ammonia the nitrogen atom being more electronegative element than Hydrogen will be having partial negative charge and making the hydrogen atom partial positive. Therefore, the attraction between these partials charges will be the main force of interaction between ammonia molecules.

Other than Hydrogen bonding interactions ammonia will also experience dipole-dipole attraction and London dispersion forces.