Who makes the best "Coaxial" Tweeter/Midrange design?

KEF perfected on that one.

BMS SEAS TAD no best just different options

You know, its been 5 + years since I've seen a pair, but I've modified a couple of different pair of cheap speakers from Best Buy... It was an Insignia  NS-B2111.  They needed parts improvements, impedance correction and some diffraction control, but man, these things ended up singing quite well. I think I paid about $60 a pair for them. My brother still uses a set with a sub as his main speakers. 

All Gradient speakers models use a coax unit made for them by Seas.

Hi OP!

You may want to take this question, and search for existing answers over at:

http://www.diyaudio.com/forums/multi-way/

You don't mention form factor. Eminence, Radian and others make 8" or larger semi-pro coaxials as well.