The best thing is a combination of solar, wind and hydro.

Hydro is king on the days when the wind doesn't blow and the sun doesn't shine. While most heat-based power plants (particularly nuclear and coal) needs long time to heat up, hydro can be scaled up from 0% power production to 100% power production in a very short time. Further, some hydro plants can be run in reverse, moving water from a lower basin to a higher basin on days with "too much" wind and sun.