I really fancy the x8 and x64 extra utils generators. I haven't really put much time in my world yet but I guess i'm starting with the x8 survivalist generators. If I understand it correctly those should produce 8x5rf/t = 40 RF/t with quite some efficiency!
For mid and late game power, Big reactors probably. Combined with railcraft boilers / turbine again to get a renewable way of creating RF.