The acidic nature of the oxides of nitrogen can be determined by the strength of the acid formed by the nitrogen oxide in water. 

More higher be the $(+) \mathrm{ve}$ oxidation number of $\mathrm{N}$-atom in its oxide more acidic be the nature of oxide. Thus ${\mathrm{N}}_2{\mathrm{O}}_5$, is most acidic oxide.