The best moderator for neutrons is the material that most effectively slows down fast neutrons to thermal energies through elastic collisions. This depends on the moderator's atomic mass and neutron cross-section. Light atoms like hydrogen are effective, but heavy water (D₂O) is superior because deuterium has low neutron absorption, allowing more neutrons to sustain the chain reaction without being captured.

Key property: Slowing down power, $\xi \Sigma s$, where $\xi$ is the average logarithmic energy loss per collision.

Final answer: heavy water