The least polluting and cheapest electricity comes from clever management of solar/wind plus storage. Storage includes dams, heat storage, batteries or hydrogen (made using renewable power).
The quickest load response times are from batteries, followed by turbines powered by dams or hydrogen. Next is gas and diesel turbines.
Worst of all is coal which is not only slow, but filthy.
The ultimate (100% battery) is not currently viable, but should be in the future as battery storage becomes cheaper and more efficient.
