Answer
$NH_4^+$ is the stronger conjugate acid.
Work Step by Step
The stronger the base, the weaker will be its conjugate acid.
Judging by their $K_b$ values, methylamine is the stronger of the pair.
$ 4.4 \times 10^{-4} \gt 1.8 \times 10^{-5}$
- $NH_4^+$ is the conjugate acid of ammonia.
- $CH_3NH_3^+$ is the conjugate base of methylamine.
Therefore, $CH_3NH_3^+$ is the weaker acid, and $NH_4^+ $ is the stronger one.
