What is (theoretically) the most efficient shape for an aircraft, assuming you don't have to carry any cargo?
The best shape for an airplane or glider would look like the ASH-30mi, an open class glider with an 86.9ft wingspan, and a 41:1 aspect ratio (wing length to wing chord). It has a glide ratio exceeding 60:1, and is considered the state of the art for gliders.