Considering the basic strength of amines in aqueous solution, which one has the smallest pK_b value?

It is known that, ${\mathrm{pK}}_{\mathrm{b}}=-{\mathrm{logk}}_{\mathrm{b}}$
Thus, larger the value of k_b, smaller is the value of pK_b and hence, stronger is the base.
The order of basic strength of amines is as follows

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

The basic strength of amines depends on the inductive effect as well as steric hindrance of alkyl groups. Hence, ${\left({\mathrm{CH}}_3\right)}_2\ddot{\mathrm{N}}\mathrm{H}$ [secondary amine] possess the smallest value of pK_b.