Power is defined as the dot product of force and velocity: $P = \mathbf{F} \cdot \mathbf{v} = Fv \cos \theta$. The gravitational force $F = \frac{GMm}{r^2}$ is maximum at the Earth's surface ($r=R$) and decreases with height. The velocity $v$ is maximum at the Earth's surface (both at projection and impact) and zero at the highest point. During upward motion, gravity acts downwards while velocity is upwards ($\theta = 180^{\circ}$), so power is negative ($P = -Fv$). During downward motion, both gravity and velocity are downwards ($\theta = 0^{\circ}$), so power is positive ($P = Fv$). Maximum positive power occurs when both $F$ and $v$ are maximum and in the same direction, which happens at the instant just before the body hits the Earth.