Probably the most efficient energy storage with current technology would be super or ultra capacitors, but they are not yet very affordable.
You're absolutely right about batteries. Besides not being real efficient, they are about the lowest cost thing going. Even at that they are expensive to buy and require frequent maintenance. There are huge batteries available that if properly cared for are designed to last for 25-30 years, but they are very large and weigh thousands of pounds.
Water storage is how some of the larger utilities do it. They pump water up to a reservoir during times of surplus energy, then reverse the process during peak demand periods, with most using the pump as a generator. Not too many of us could afford that type of operation, and the overall efficiency is very low. There are a lot of losses in both the pumping and generation phases.
Too bad fuel cells aren't more efficient. Generating and storing hydrogen under high pressure is pretty easy. Getting energy back out of stored hydrogen is a different matter. For conventional parahydrogen (H2), which is what stored hydrogen would become after the orthohydrogen decays, burning in an internal combustion engine averages about 20% efficiency. Lower cost fuel cell efficiencies are typically around 35%. If one is wealthy enough to buy exotics, they are usually around 55% or so.
I like the flywheel idea. Spin up a massive weight with surplus energy, then use that stored energy to generate when needed. Motors and generators can be built much more efficient than many other methods of energy storage. Let inertia be our storage medium.
Bob