Well ... all over the world where there are canyons and when/where upper and lower reservoirs can be built into them - and when/where evaporation can be replenished in those reservoirs as needed ... has always seemed to me to be best for energy storage in that it duplicates Mother Nature's energy storage method. Pump those reservoirs full using renewable energy when it's available, and then drain them through turbines to generate electricity when renewable energy isn't available.
That approach for massive energy storage sure seems "simply the best way" to me - when things get desperate enough for mankind to become really serious about the need.
Of course transporting the resulting electricity all over the world to places where there aren't any canyons - preferrably without using wires - probaby still requires massive electromagnetic wave research.
