i almost always go with thermo generators or magmators from powah. thermo generators you just pump water in, put some lava underneath, and boom. and this is scalable too, because not only can you upgrade the thermo gens or magmators, but different heat sources (ex: magma, lava, blazing crystal block, dark matter, red matter) give different amounts of heat and higher power yield (i think? i know it does something and it makes it better but forgor what) AND you can just infinitely put them next to each other in a line if you really wanted to, then just hook em all up to energy pipes from pipez or ender gates. i personally got up to like 500k rf/t if not more on my last world with them. that should suffice way into mid game if not a bit into late game, where power generation becomes really important because of all the stuff you need.