The best speaker you ever heard?

Hands down, the Infinity Reference Standard II A system. Back in 1988. Nothing like them ever since. Nothing else has even come close, but surprisingly enough my Bose 901 Vs deliver, in their own limited way, a pleasing result that surprises sometimes the same way that Infinity system did.....
:-)

Since numerous responders have mentioned Sound Lab electrostatics as their choice, I would like to add that they have only continued to improve over the years. The latest improvement is the Toroid II transformer in the back plates, which has increased their efficiency and resulted in a noticeable step-up in everything they were always acclaimed for.

I worked at Radio Shack for 5 years or so back in its hifi heyday. Those were my least favorite popular RS speakers. Tried to talk people out of buying them, except maybe when a good sale ran. They always sounded like all the bad stereotype things you hear about horn speakers, boomy bass, shouty and shrill. Maybe with the right tube amp that RS no longer sold. That was the heyday of the SS receiver/amp, mostly made in Japan.