As with any "What's the best..." poll, it depends wildly on your needs, materials, brainpower, ambition, and more.
My current world has a setup of three BR Turbines being fed steam from MFR Steam Boilers (five per Turbine). Those Boilers are running off the water output from the Turbine and Lava buckets being provided by a Lava Fabricator/Fluid Transposer combo. These machines are powered by the Turbines themselves, creating a self sustaining positive RF cycle.
Prior to this, I had our small server powered solely by mob farms. MFR Grinders feeding gunpowder and mob essence to TE Reactant Dynamos was the basic power, with a ton of other mob drops being used for other power generation methods. Those included: potions in a XU Potion Generator, Blaze Rods in Furnace Generators, Ender Pearls in an XU Ender Generator, rotten flesh in a Culinary Generator, sticks in Survivalist Generators, Redstone into a Magma Crucible then into a Heated Redstone Generator, and more.
After a while, a friend took to using sticks, string, and bonemeal from my farm to automate MFR Industrial Fertilizer for his Agricraft farm. He used nine crops at 10/10/10 into a MFR BioFuel Generator into seven BioReactors. Later he upgraded to use four RC Liquid Fueled Firebox Boilers to power a BR Turbine.
So far as easiest early game, I usually use either a Survivalist Generator or an Ender IO Stirling Generator to start with basic needs, then spiral out of control on some random path that I'm told can't be done. No greater motivation on Earth than "I bet you can't..."
Edit: If you want a fun puzzle, try to power your world off RFTools Enderic Generation (probably not actually what its called).
@McJty can correct me, I'm sure.