Is the most efficient speaker the best speaker?

All other things are never equal so its a useless question unfortunately.

More efficient speakers are more efficient and that's about all you can say about that.

Most portable electronics that operate out of necessity on low power are very efficient. DO they all sound good?

case closed.

Touche'!
Some times one can use the basis of "all other things being equal", but I don't think it works at all for loudspeakers, not for the time being anyway.

All the speakers that I have owned and liked best have been sealed and they are less efficient than most others. To my ears a sealed woofer gives the best bass of all other designs.

I haven't read all of these post, so it might have been answered...The answer to the exact question is yes.... In theory. If all things are totally equal except for sensitivity, the two speakers will sound identical except in the area of dynamics. The more sensitive speaker will be more dynamic. Next... This is impossible

I've read though the post... The op says "All things being equal" no debate on horns vs planars... Two identical horns, playing exact frequency curves, playing exact impedance & phase curves, playing the exact music... Or two exact matching planars or two exact matching dynamic drivers.... ALL THINGS BEING EQUAL, The more sensitive speaker will be more dynamic... The answer is Yes.