The torque and magnetic potential energy of a magnetic dipole in most stable position in a uniform magnetic field($\bar{B}$) having magnetic moment ($\bar{m}$) will be.

Torque, $\bar{\tau }=\vec{m}\times \vec{B}=mB\sin \theta$
and magnetic potential energy $U_m=\vec{m}\cdot \vec{B}=mB\cos \theta$
at $\theta =0^o$ the dipole will be in most stable position
$\tau =mB\sin \theta =mB\sin 0^o=0$
and $U_m=-mB\cos \theta =-mB\cos {\theta }^o=-mB$