Is the most efficient speaker the best speaker?

Unsound- certainly!

On some level I agree with you, I just want to know at what point does pursuing this one consideration of the many, lose sight of the total goal.

That is a matter left to the speaker designers! I can say though that they have been getting some impressive results in that regard in the last decade or so, so much that they won me over- before I got my current speakers I really did not think horns were 'high end'. Boy was I wrong.

I have yet to hear any examples to win me over.

I don't understand how a 4" driver, even if has high efficiency due to horn loading, can move enough air to have much dynamics. A 12" driver has TEN times to surface area, not to mention much, much higher Xmax.

(Forgive me but I have not read every post in detail)

"I don't understand how a 4" driver, even if has high efficiency due to horn loading, can move enough air to have much dynamics."

Its all relative and depend on room size but basically teh answer is it can't compete in an apples/apples test.

Unsound, even if you consider them deficient, what are the best sounding horns you have heard?

The best I have heard are custom GoTo horns. I did not get to hear them over an extended period of time, but what I heard was quite good and with my eyes closed I do not think I could have guessed they were horns other than the fact that the dynamics were superlative.

Mapman, all thing considered, probably the Klipsch Forte.