The force is greatest when the wire is perpendicular to the magnetic field.
In fact, the expression for the magnetic force is
[tex]F=ILB \sin \theta[/tex]
where I is the current, L the length of the wire, B the magnetic field intensity and [tex]\theta[/tex] is the angle between the directions of I and B.
We can see that F is maximum when [tex]\sin \theta=1[/tex], so when [tex]\theta = 90^{\circ}[/tex], so when the wire and the magnetic field are perpendicular to each other.
