My choice for transient response with all the other desirable things included is a toss up between...VMPS tower II special editions...no longer made and seldom ever show up for sale used because people that buy them, keep them forever. Ribbon super tweeter, inverted cone tweeter, 5 " carbon fiber midrange and Dual 12" active drivers with a slot loaded passive radiator at the bottom of the enclosure.
And then the most amazing sound I have ever heard at any price, Ohm A - transient response and distortion extremely low at ALL frequencies. Truly an amazing loudspeaker, but ultimately flawed, limited ability to play loud without self destructing from cone over excursion and they required like 300 watts minimum to sound good and 301 watts to blow em up. I would love to have a speaker with the Ohm A sound quality but better durability and better dynamic range.
3rd on my list is the Klipschorn for all its capability, it is just not that accurate but it is exciting and engaging to listen to... and I think it has the transient response and relatively low distortion characteristics that are required.
I owned the VMPS for 20 years before a fire destroyed them...I cannot find anything so far that I like as well, let alone better. I wanna try out the Ohm F5 which is said to outperform anything Ohm has ever built. IF that is true, that might well be my endgame speaker. We are gonna find out in a few months....