Hydro. You turn the tap on. Nothing is faster. They do it at times of high demand when the price is high, and turn off when demand and cost falls.
Pumped hydro, they pump water up to the reservoir when demand and price are low, and run it down when high.
They can predict demand pretty well. You will find the AC frequency varies by only a very small amount with mismatch of S and D.
Oh yes, I am well aware of hydro-electricity and pumped water storage, but both of these require very specific locations - mostly in Scotland and Wales, if I'm correct. I still don't think they could replace the ability to provide an instantaneous output when needed that is presently done by fossil fuelled power stations.
Metered Windpower currently produces in the region of 5%-10% of UK supply and, once built, will sell into any price. Last Saturday it was producing almost 5GW, and as I write it is about 1.6GW. Unmetered windpower is about half as much again.
Total demand as I write is about 32GW and Nuclear is producing about 7.2GW
Wind power is unreliable as it (obviously) requires wind! Not only wind, but wind that is neither too weak or too strong. We don't want blades flying off and generators burning out!
Nuclear power, on the other hand, can provide a continuous output. The only drawback is that we haven't invested in it sufficiently. The Frogs have, and consequently we are obliged to buy electricity from them.