If a loop of wire with an area A is in a magnetic field B, the magnetic flux is given by:
Flux = B A cos theta ( where theta is the angle between magnetic field line and perpendicular of plane.)
EMF = d Flux / dt
When plane is perpendicular to magnetic field the angle between perpendicular and plane is 0 degree.
Cos theta is max ie 1 when angle theta is 0.
So EMF will be max when theta is 0 ie when plane is perpendicular to magnetic field, the EMF will be max.