Earth is flat at poles making them close to the centre and there is no effect of rotation of the earth at poles.Hence maximum g there.Centre is surrounded by symmetrical mass distribution cancelling the effect of each other.
since the earth rotates at a very high speed so centripetal force acts on it to great extent.
so the matter along the equator generally tends to move away from the centre and there is comparatively less effect of centripetal force at the poles.
so comparatively poles are closer to the center than the equator and according to the universal law of gravitation force is inversely proportional to the square of distance between the two objects.
So, it is greatest at the poles as it is the closest.
Now, it is minimum at the center of the earth.
Take the earth as 2-D for a moment. You get a circle. So the object on crust means object on circle. Now the whole circle is taken into consideration. More mass more force and hence more force at crust.
Now when you go deep into the earth suppose go half way. Now the crust for you will be that halfway not the crust that you were previously standing upon. So now the earth for you will be the smaller circle as i have drawn in the attachment. The yellow region will have no role to play now. The area in pink is the earth for you at the final point.
So since the mass of earth is decreased now, the force will be less.
earth.jpg