Best speaker brands for transient response

Transients are impulse responses in relation to steady state and it can be measured. Impulse response is to a specific input from base level. It is used to check time alinement. It is not related to distortion but most systems with good transient response are also low distortion. It comes down to good power to weight ratio of the drivers with pistonic motion.  
 

A system that can make a square wave at the desired SLP in theory has perfect impulse/transient response. Most amps have no problem with this and most all speakers fail this test. I have seen headphones (single driver, no crossover, low output) pass. 
 

This sounds about right:

 In the context of high-end audio, the term transient response refers to the ability of an audio system to accurately reproduce quick, abrupt changes in sound, such as the attack and decay of musical notes or the leading edge of a percussive sound. A transient is a short-lived signal that contains a high amount of energy, and a good transient response is important for accurately reproducing the natural, lifelike quality of music.

A high-end audio system with a good transient response should be able to start and stop quickly, with minimal overshoot or ringing, and should be able to reproduce the subtle details and nuances of musical performances. This is in contrast to a system with a slow or sluggish transient response, which can make music sound dull, lifeless, and lacking in detail.

A variety of factors can affect the transient response of an audio system, including the design of the amplifiers, speakers, and other components, as well as the quality of the cables and interconnects. In general, a high-end audio system that is well-designed and well-implemented should have a good transient response, allowing it to accurately reproduce the dynamic and nuanced characteristics of music.

Interesting that most the OP's description of transients relate more to midrange and high frequencies, not bass.

"The target effects I use to test transiency are the crash-to-ring transient of a cymbal, fingers going across guitar strings, snare drum tightness.."

Based on this, I suggest it is not a question of bass alignment  (sealed vs ported) but rather the mid and high frequency performance that will provide the desired effect. Certainly the room and speaker placement can introduce reflections that can cause smearing in this frequency range. 

An interesting characteristic of dipole speakers (Magnepan, some electrostatic, open baffle) is that they interact with the room differently. Yes you may need to absorb the rear radiationwith suitable room treatment, however they do not radiate to the sides or the ceiling. I find this can give a clarity to the mids and highs that is very appealing...and the lack of a box can provide a very natural and uncoloured bass. Worth exploring.

I have the Magnepan 1.7i and enjoy them...I think you would find they have a good transient response.

Transients are impulse responses in relation to steady state and it can be measured. Impulse response is to a specific input from base level. It is used to check time alinement. It is not related to distortion but most systems with good transient response are also low distortion. It comes down to good power to weight ratio of the drivers with pistonic motion.

When it comes to transducers, “transient response” is how close a driver can mimic the original signal. If given a theoretically perfect amplifier and speaker enclosure, any deviation from perfect transient response is indeed driver distortion. The more imperfect the response, the higher the distortion.

You are correct that lower mass diaphragms tend to have lower distortion, especially if they can resist bending modes within their implemented bandwidth.

Sometimes a driver can have good piston behavior but still cause a comb-filtering effect, which is another, typically less subjectively offensive distortion. Regardless, any deviation from perfection is a distortion.

The reason higher quality drivers sound quicker, subjectively, is because they are lower in distortion.

Ported, sealed, OB, panel…all such designs can sound slow/fast or dynamic/anemic depending on how well they are designed and the quality of their transducers. Unfortunately, most dealer-sold tower speakers under ≈$7K/pair (and stand-mounts under ≈$4K/pair) employ very mediocre transducers.

What is a transient response?  I have not heard of that term before.

This link gives a good overview to augment @helomech's explanation.

https://www.hifireport.com/understanding-transient-response-in-audio-a-beginners-guide/?utm_content=cmp-true