The strongest base among the following is

Secondary amines $\left(2°\right)$ are more basic than tertiary $\left(3°\right)$ amines. This is due to steric hindrance effect amines in aqueous medium. The decreasing order of basicity is follows as below.

${\left({\mathrm{CH}}_3\right)}_2\mathrm{NH}$$>$${\mathrm{CH}}_3{\mathrm{NH}}_2$$>$${\left({\mathrm{CH}}_3\right)}_3\mathrm{N}$$>$$\left({\mathrm{C}}_6{\mathrm{H}}_5\right){\mathrm{NH}}_2$