Answer
$NH_{3}$ is the stronger base
Work Step by Step
A good strategy to solve this problem is by comparing the acidity strengths of the conjugate acids of both bases. The fluorine atom in $NF_{3}H^+$ has a greater electronegativity than the hydrogen atom in $NH_{4}^+$, so $NF_{3}H^+$ loses its proton more readily than $NH_{4}^+$ due to the stronger inductive effect in $NF_{3}H^+$. Therefore, $NF_{3}H^+$ is a stronger acid than $NH_{4}^+$ and on the other hand $NF_{3}$ is a weaker base than $NH_{3}$.
