Imagine a sail on a boat; it catches the most wind (receives maximum force) when it is set at a 90-degree angle to the direction of the wind flow.

FIB (Force, Induction, B-field) $\rightarrow$ Force is max when current and B-field are at $90°$ (perpendicular).

The magnetic force $F$ on a conductor of length $L$ carrying current $I$ in a magnetic field $B$ is given by the cross product $F = I(\vec{L} \times \vec{B})$. The magnitude is $F = BIl \sin(\theta)$, where $\theta$ is the angle between the current direction and the magnetic field. Since $\sin(90°) = 1$ is the maximum value for the sine function, the force is maximum at $90°$.

Remember that magnetic force is a vector cross product; parallel vectors result in a zero cross product ($0$ force), while perpendicular vectors produce the maximum product.