I'm finding lava as a fuel source very efficient. If you use a netherrack fire under your crucibles rather than torches, they process the cobble three times as fast compared to torches (I got that info from an Ex Nihilo wiki), which means that 3 crucibles are sufficient to meet the demands of two magmatic dynamos running at full power. As a crucible processes a piece of cobble in 40 seconds that way, a single Igneous Extruder can keep up with the demands of 12 Crucibles, and those, in turn, can power 8 magmatic dynamos. 640 RF/t with early-game resources, no farming and almost nonexisting logistics. I don't think there is anything as space-efficient, counting the total you need to make the fuel as well as the power generating facility, until you get to the Big Reactors.