I just cut from foam and go fly it to see if any of the theory is relevant to small models .
The textbook answer to most efficient wing always ends up as a great glider wing --and a lousy low speed aerobatic wing.
Lift is drag and drag is lift . call it whatever you like .
for some planes , the most efficient wing is one where you can trade off high lift for high drag - and still retain high maneuverability and transition from one to another.
For those who don't fly --I mean a setup that has lift at very low angles of attack and instant braking power (all drag )- if suddenly angled sharply
No good for full scale DC3 but GREAT for a high powered low wing loading model aerobat.
This may be a textbook somewhere - probably in Russian --