The most dynamic speaker you've heard...

My giant conical horns but there over 12ft h about 1200lbs ea and 110db 1 watt

I haven't found any speaker that has exhibited exceptional dynamics under all conditions. Great horn drivers are truly exceptional at lower volume levels and can make music sound lively and engaging at surprisingly low levels, but, they tend to "shout" a bit when pushed to higher volumes. A lot of the the dynamic speakers people have been listing above are good at higher volumes, but sound lifeless when played at softer volumes. The same goes, to me, with the MBL speakers--great at higher volumes, but not the best at lower volume.

Some horn and field coil speakers, like the Classic Audio T-1, sound very good across a surprisingly wide range of volume.

I rarely listen at high volume levels so I have gone with fairly high efficiency speakers that exhibit great dynamics at lower volume. My midrange is a Western Electric 713b driver, the woofer and tweeter are modern high efficiency models. The 713b is a fantastic driver, but, it does exhibit a bit of "shouty" quality when played at high volume (not dramatically so, but noticeable). Everything in audio involves so kind of tradeooff, so personal priorities are important.

Seems like there is a little bit of clarity needed when discussing this topic. I have seen only in one place the specs of sensitivity and efficiency. If you can unerstand one speaker, a raw driver had an efficieny of 6%, and a sensetivity of 97db (?). Some horns are as much as 25% efficient, which is way more than just the driver. Not a lot of this stuff is actually published from what I see.

I driver whith a sensitivty nearing 100db in a horn should be able to deliver the most impressive dynamics an most any loudness level. Unless the program material has been compressed, that is.

Just based on principle, hi-eff drivers like horns and line arrays should do it. Room size matters. Danley designs should be something to think of?
However, just dynamics is not the answer to music fidelity as one should know. A good part of the equation, however, there are too many variable out of the field of view of this topic.

In a small room, Cessaro Horn, Chopin. Have only 11" drivers but sounded fuller and more dynamic than the Sasha!