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.
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.