Small Fish 64 Stack Tree Farm, 72% Efficiency
This small fish-shaped tree farm is designed to provide the farmer with a regular stack of 64 wood blocks (which I estimate requires 13-15 trees; this design includes 18 for good measure). It is small, space efficient, and aesthetically appealing. Note that, as usual, the ceiling should be 8 squares above ground level to keep all of your wood within easy reach. I recommend constructing the exterior using colored wool in a colorful aquatic theme.
Rewards and Conclusion