The most dynamic speaker you've heard...

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!

All the technical info and how it relates to a speaker sounding dynamic is great, but I find it very interesting that you equate "dynamic" with "the sound of a pretty soprano piercing my [your] heart". That is a perspective that I can relate to. The sound of a pretty soprano is not the musical example usually used to discuss dynamics; if one is used at all. When a speaker sounds alive it doesn't matter wether it is particularly extended (in either direction), nor is necessarily able to play very loud. The problem that I have found is that many of the most alive sounding speakers that I have heard (usually horns), are not very "pretty" sounding tonally. Since assembling a satisfying system is always about compromise somewhere in the chain, and mixing tonal flavors (wether we want to admit it or not), I have found the solution to be a reasonably dynamic speaker (not necessarily the "most" dynamic) with good tonality well matched to tube amplification which to my ears tends to have more of that elusive "alive" sound than most ss.

Having said that, the "most dynamic" speaker I have ever heard was the Jadis Eurythmie; but I hated how they sounded tonally. Give me electrostats with well matched tubes any day.