Is the most efficient speaker the best speaker?

"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.

Unsound, what was the rest of that system like?

I've head them in many, mostly modest systems, though not in some time. The reason I mentioned them was; they were inexpensive speakers, that didn't take up too much space, that could be driven by a cheap receiver, and still fill a large space with nearly full range music without completely embarrassing themselves. Very few others could do that, without costing significantly more (percentage wise). Not my cup of tea, but could work for some others with a large space, on a tight budget, that didn't want overly large boxes dominating the room, and not interested in too much fussing.

Yes the biggest problem I have with horns is the ones that sound really good to me are big and expensive at which point they tend to lose their benefit overall for me.

If I could fit the better ones I have heard and afford them as well, they would be a viable option that I would strongly consider.

I've also considered Forte's in that they are not overly expensive, get good accolades, and actually might fit into my house somewhere someday. But I do not expect them to become my #1 go to speaker.