From the side/angle inequality, we see that the smallest angle of a triangle must be opposite its shortest side. In this case, that angle is opposite the shortest side [tex]\overline{AC},[/tex] so our answer is [tex]\boxed{\angle B}.[/tex]
As for finding its measure (which I'm aware the question probably didn't ask for), we can use the law of cosines:
[tex]5^2=6^2+7^2-2(6)(7)\cos B[/tex]
[tex]\cos B=\frac{5}{7}\implies\angle B\approx\boxed{44.4^\circ}.[/tex]
