Is the most efficient speaker the best speaker?

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.

Ptmconsulting, that was very well said, and two good examples. Proper amp/speaker matching is a fundamental key to sucessful system building.

Keep in mind we listen to a system not just a speaker, source, cables, amp etc. Also we all have different personal tastes and expectations. So while I prefer efficiency even when using higher power. Others may prefer a stat or dynamic driven with hi power.

I think the OP is asking a hypothetical question

Not to mention personal preferences. Both in what aspects of music you listen to and your musical preferences. My own prefs lean to high eff, in part because of some of the great small power amps that can be run with them. Cost was certainly a factor too (though you can have megabuck or inexpensive systems with either set-up)