Is the most efficient speaker the best speaker?

I think with all other things equal, then I say yes it is best to have high efficiency speakers so the amp can cruise along in an effortless mode and for the right solid state designs class A all the time.

why not?

Many varied factors come in to play. A well designed high efficiency speaker has the inherent advantage of increasing the pool of usable amplifiers. This opens the door for the inclusion of low power(high quality) amps such as SET,DHT-push pull, first watt etc. These types of amps with an appropriate speaker can sound fantastic. A good high efficiency speaker in general tends to be very dynamic and immediate even at moderate/lower listening levels. Just as with the less efficient speakers, some are better implemented than others.

Maybe yes, maybe no. But SETs are the best amps, and the first watt sounds best. So you're going to need efficient speakers, if you want the best sound IMHO.

Is the most efficient mode of transportation the best way to go?

I've heard amazing systems with 3 watt flea amps + Klipshorns

and I've heard amazing systems with 1000 watt monster amps and 83db inefficient Apogees

it's all about synergies but how you get to the sound you like can take multiple paths