As among N,P, As and Sb, the former has highest electronegativity (EN) so its oxide is most acidic. As the electronegativity value of element increases, the acidic character of the oxide also increases.
The oxide with the highest positive oxidation state on the element other than O should be most acidic. Oxidation state of V in V2O5 and N in N2O5 are same. But the electronegativity of N is higher, making N2O5 the most acidic oxide.