there are several ways of producing electrical energy (coal, hydrpower, nuclear power, solar, wind, geothermal, etc.) but which way is the best"?

Well, that depends on cost, maintenance, and power output most of the time. Geothermal seems to be best because you could heat up water, and the heat from said water can be used to create electricity. Making it extremely eco-friendly, which makes it the best in that regard.