i hate one ply, why to they even make that... personally i would say 2 ply is the standard, if you are lucky enough to find 3 ply well then... you are in luck....
4 ply is standard. 6 through 10 ply are for heavier and heavier loads, which will be more than our STs are rated for. The higher ply tires will ride harder, as they are made to handle heavier loads. I had a F250 with 10 ply tires that I used for hauling wood. I'd load it down with green(wet) oak, and the tires could handle that weight, where a 4 ply would have had problems...
The answer Chris provided is correct, but it relates to weight capacity. How does that ranswer your question about to efficiency???? All tires regardless of ply have different efficiencies. The ply only relates to the load carrying capacity, thats why you were asked several times to clarify your use of "Efficent"