high-efficiency loudspeakers

While there is no "best", I will say that Legacy Audio comes close..I've heard most of the above mentioned brands, and Legacy is the one that captures my heart and soul. A very efficient speaker not needing a lot of power to drive, but at the same time will handle a huge amount of clean power. Even the Legacy bookshelf speaker (Studio HD's) have a awesome sound, and have sound much larger than they actually are...they also score well on the wife factor, with their custom finishes she will find something that will fit her taste..the Rosewood finish did the trick for my better half.
www.legacyaudio.com

@bbarlow The Spatial Audio X2's look very promising indeed, and they could turn out to be real contenders.  I'll be watching the thread on A. Circle carefully.

Twoleftears:  I promise to report back.  Unless I keel over from their wonderfulness...

There are several manufacturers of hi-efficiency speakers, nut not many that are commonly discussed on this forum.  There is no "best", just what's best to you.  Mating a speaker to an amp for best sound involves a lot of factors besides the sensHere are some more manufacturers:
- Audio Note E,J, K or their DIY counterparts from ANK kits
- Avantgarde Acoustics - extremely efficient and pricey
- Bob Crites horn designs - the Cornscala will change the way you think about horns
- Cain and Cain - single driver speakers
- Classic Audio speakers
- Coherent - made in Canada.  Look beautiful but I have not heard them
- Coincident
- decware - all great
- Devore O/97 and O/93 - outstanding speakers
- Omega - the best single-driver speakers I have personally heard
- Pi
- Reference 3A - you will see a lot of PP amps using these
- Rethm - the Bhaava is an amazing, affordable point source/rear loaded horn speaker w/ an active bass driver
- RL Acoustique - Lamhorn
- Shindo - upscale Klipsch designs, sound wonderful but $$$
- Silverline - some models
- Spatial - the M3/M4 Triode Master versions.  Quite affordable
- Trenner & Friedl - some models
- Tyler Acoustics - some models
- Volti Audio - 21st century reworking of the classic Klipsch horn designs
- Tekton & Zu - not my cup of tea but a lot of guys love these

I may have missed some.  There's also a whole world of older/classic speakers from an age when high-efficiency was a requirement and not a choice.  Here are a few suggestions:
- Altec Lansing - Specifically the models 14, 15 and 19
- Cain and Cain - single drivers, highly regarded, company recently demised
- KEF
- Klipsch - all classic Klipsch models w/ horns are very efficient
- Snell E, J, K or the /II models - the genesis of the Audio Note line.  If you want to see if AN speakers will work in your room for cheap, grab a pair of these in good condition
- Tannoy - some models, although I always found these to be much less efficient than listed by the specs 
- Von Schweikert db99 speaker - a recent model from a current mfr

I'm sure that I missed someone's favorite speaker vendor.  If so my apologies...

The real test for a high efficiency speaker is if it can be powered by an 1W amplifier and reproduce the entire frequency range (including bass) with 1W or less. Here is one review that covers such a system: http://theaudiobeatnik.com/linear-tube-audio-mz2-s-originally-released-microzotl2-0-teresonic-magus