Final answer:
The rate at which water flows over Niagara Falls is best described in cubic meters per second (m³/s), as it is the SI unit for large flow rates.
Step-by-step explanation:
The most appropriate unit to describe the rate at which water flows over Niagara Falls is cubic meters per second (m³/s). This is because the flow rate of such a massive volume of water is best represented in a unit that conveys the scale appropriately. The SI unit for flow rate is m³/s, which is a commonly used metric unit when discussing large flow rates such as rivers or waterfalls.