Depending on what exactly you want to measure, the largest grid storage by far is the power-to-gas/gas cavern/gas power plant cycle. But of course only a minisc

Depending on what exactly you want to measure, the largest grid storage by far is the power-to-gas/gas cavern/gas power plant cycle. But of course only a miniscule fraction of the energy currently going through caverns and gas plants comes from power-to-gas, so it's not wrong to put pumped hydro ahead (possibly even battery installations, p2g is tiny)