A magnetic dipole in a constant magnetic field has

When a magnetic dipole of dipole moment is placed in a uniform magnetic field, it will experience a torque,$\tau =MB\sin \theta$

Torque is maximum when $\theta =90°$

${\tau }_{\max }=MB\sin 90°=MB$

Potential Energy of a magnetic dipole in a uniform magnetic field is,

$U=-MB\cos \theta =-MB\cos 90°=0$