high-efficiency loudspeakers

I'm very happy with my single-driver Omega Super 3i Monitors. Fantastic speakers at their price-point. Efficiency is 94.5 dB at 8 ohms and they're mated right now with an inexpensive Chi-fi (3.5 wpc) SET integrated. A Decware SE84UFO (2 wpc) is in my immediate future. 

Have Zu Definition IV, and with the right electronics [tubes] they make music. Will say that with any hi-eff speakers you have to clean up the AC a bit. Actually, I'm in love with them but hate to do the fanboy thing.

best in hieff is full front loaded horn systems

I've found that a pair of recently acquired Klipsch Heresy IIIs are much better than I expected (not sure what I expected in my house, but I did force myself to listen to a pair at a dealer before before buying these damn things) and since many haven't heard them since 1957 and might think, as I did, that they're an anachronistic old fashioned pile of beatnik era "hornist" crap, you may find they're pretty friggin' great. Or maybe you're DEAD INSIDE.

If there is a 'best'... I look at what would be on the short list.

Another vote for Classic Audio Loudspeakers. 98 db 1 watt/1 meter, 16 ohms, 20Hz (honest) to 35Khz). Optional field-coil powered woofers and midrange; first breakup at 35KHz so very smooth and very fast.