What is most efficient way of storing mechanical energy?

You can store it as potential energy or kinetic energy or a combination of both. I think best way should be by seeking help of gravity. Use the available energy to lift some mass to a given height. read more

One way to store mechanical energy is to convert it into potential energy, such as raising a weight. Many grandfather clocks use this method, and it can store energy for an unlimited amount of time. In a similar way, you can store energy in compressed springs. read more

Another way to store energy is in some form of repeatable mechanical deformation. This is the idea behind a spring used in a wind-up clock or a rubber band used in a wind-up airplane. You store the energy by bending (deforming) the material in a spring, and the material releases the energy as it returns to its original shape. read more