Is the most efficient speaker the best speaker?

Maybe yes, maybe no. But SETs are the best amps, and the first watt sounds best. So you're going to need efficient speakers, if you want the best sound IMHO.

Is the most efficient mode of transportation the best way to go?

I've heard amazing systems with 3 watt flea amps + Klipshorns

and I've heard amazing systems with 1000 watt monster amps and 83db inefficient Apogees

it's all about synergies but how you get to the sound you like can take multiple paths

All other things are never equal.

My experience has been the opposite, but the answer to your question is no. My Soundlab speakers are very in-efficient, but are the best sounding speakers I have owned.

The key is the right, properly matched, amp and speaker combo!

No way to say one speaker will sound better then another based on efficiency only.