What is the most energy efficient siding?
I don't think that I would pick siding based on energy efficiency. I know that some vinyl and aluminum siding is being sold on the basis of some insulation board which is installed under the siding. This is usually a relatively thin insulation material and produces low benefit (as related to the total cost of the project).
Also, some recent problems with stucco-like siding products are a result of moisture entry between the insulation board and the sheathing.
In new construction, the insulation should be inside the wall.