There is of course no definative answer !
One first has to ask the questions: size, weight, power, speed ?
For the most eficient, look at the development of thermal soarers, be they models or full size.
But in the modelling world one has to ask: what are we trying to achieve ?
Scale, looks, handling charicterists. Or do we just want a model that will
survive the rigors of many flights and being stored in the shed etc.
A model that can be carried on the back seat of your Reliant Robin
Its been proven, a lawn mower will fly if the power to weight ratio is right
Each to his - her own.