[sliverstorm]

The best bang for buck is always plants. Trophic levels are typically ~10% efficient, meaning an herbivore that consumes 1 BTU per hour needs to eat 10 solar BTU per hour's worth of plants. The same generally holds as you go up trophic levels- which means 1,000 BTU of sunlight provides a 3rd trophic level predator with 1 BTU.

Though, this is an ultimate efficiency answer. For short-term small-scale, herbivores/predators could be a better answer, because they gather energy from all over the place, so harvesting them is sort of like harvesting plant life from your entire block.