high-efficiency loudspeakers

I'm surprised no love on here for the Devore 0/96. I haven't heard them yet but would love to.

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.