high-efficiency loudspeakers

Best? There is no such thing. There are too many variables for any absolutes. I'm surprised that nobody has mentioned the Quad ESL 57. There was a recommended 25 wpc maximum on those if I remember correctly. Sure some will like horns, personal preference and all that.
After having many different types and styles I settled on VPMS Tower II many years ago and have never looked back. I picked up a second pair and can drive both pair with a 20 wpc tube amp with glorious results.

Best? There is no best. It's all in what you like.

I had B&W 804s. I thought those were efficient. Then I got the Tekton DI upgrades. Made the 804s sound bad and inefficient in comparison. I now have the Tekton Ulfs. More efficient and even better sounding than the DIs. I think I've slain the speaker dragon. I'm not even looking anymore. Not a very Audiogonish statement, I know.

The Klipsch KLF-20. I bought it brand new back when they were out in light oak and I still have them. It's not the only speakers I own, but these KLF-20's will be with me for awhile.

@kqvkq with regard to Quad ESL 57s the recomended maximum amp size was 45 wpc as I had them and when Quad brought out their 405 current dumping amplifier they stipulated that you had to put current limiting resistors in it to limit the amp to 45wpc when using the 57s. I'm afraid I know this to my cost as twice I had to replace bass panels as I thought I would be smart and not use the resistors.

Ken Shindo gets the hammer green color that he uses in some of his amps from the Altec Lansing 604C. 

He actually once used a pair of 604Cs in a pair of floor standing cabinets that were being sold for $33,000 in a Manhattan audio salon, maybe two summers ago. 

The pair I have are in an identically sized pair of cabinets. I added the Mastering Lab crossovers, the whole package costing me $3,300, a mere tenth of what the “Shindo” pair eventually sold for, to some unsuspecting Wall Street sucker no doubt. 

I’ve had my pair for 35 years. Every 7 or 8 years or so I figure there must be something better, I go shopping for a month or so and decide well, yes, this or that may be better but is it $50K better, and the answer is always decidedly no. 

The speakers are 103DB. I run them with a pair of Julius Futterman OTL3s, converted to triode. The depth and detail are outstanding. 

I’m basically a happy camper.