The gravitational field is maximum at the poles. The earth is spherical in shape; it is  slightly flat at both the poles.

• The radii of the poles are smaller as compared to that of the equator and since, gravity is inversely proportional to radius, this makes the gravity maximum at poles. 
• The centrifugal force, which is the outward force of the mass, is zero at the poles. Thus, it helps in maximizing the gravity at the poles. 
• The earth has more mass at the equator because of more radius, thus, gravity is affected. This is not the condition at the poles, hence, there is more gravity at the poles.

Hence, the gravitational field is maximum at the poles of the earth.