Led's are incredibly efficient for lighting. I use strings or Rope lights to light my house.
but your question kind of implies that you have no power. for that I would have to go with candles of some kind, really big candles burn very slowly so might be what you need.
Nah, I've got power. I, just curious what things are actually most efficient without all the externalities. I can see LEDs being way more better than CFL or other light bulbs because of how much less stuff they use. Its like just a wire in a tiny blog of glass.