The force is given by F=iL×B where i is current, L is the path the current follows (the wire path) and B is the magnetic field. Also recall that the × symbol is the vector cross product, which can be expressed by the sine of the angle between the vectors L and B. Therefore we can also say F=iLBsin(θ). Do you know what value of θ makes sin(θ) the largest? This will tell you if parallel or perpendicular makes the force strongest. For one value of θ it will be zero, and for the other it will be maximized.
