The best speaker you ever heard?

So far the Wisdom Audio D-75 and they make a larger Model the RUSH.

Apogee .... all of the Apogee full range ribbons are frontrunners, but the Diva, followed by a Duetta Sig, cross the finish line first.

According to Greg's suggestions, which I find excellent, I should have mentioned the Acapella Exaliburs.

I am intimately familiar with the Wisdom 75's, both in their original and second generation. They are not an easy speaker to find an amp for. The woofers are very current hungry and the planars are quite revealing of amp character. Well set up and properly driven, they can create a huge layered stage and be quite dynamic with thunderous lows, at least above 30 hz. Having said this, I have yet to hear a pair that has the delicacy, immediacy and transparency of a well set up pair of Quads being driven by a good tube amp. This does not mean that the Quads are better or worse, only that they have very different strengths and weaknesses and that neither is perfect.

Fcrowder, the Quads driven by a pair of Jadis 80 monos, HP of TAS once called a match made in heaven and by golly, how right he was!