The largest deflection of alpha particles in an electric or magnetic field typically occurs when the particles have lower energy and are interacting with particles that have a low mass number, as this leads to greater relative interactions with the field.
So, the best configuration for the largest deflection of alpha particles would be:
low-energy alpha particles passing through a field of low mass-number particles.
This is because low-energy alpha particles would be more easily influenced by the electric or magnetic field of lower mass-number particles.