What speaker has the best high frequencies?

I’ve tamed the Heils and they sound great with plenty of top end extension. I use them in a 2.5 way with JBL 2241H 18" and JBL 2251J 10". My own design crossovers.

I created acoustic lenses for them and insert a piece of foam to the rear to reduce reflections improving imaging. Here you see the RTA on the couch where I sit. There is no eq or digital processing of any kind being used here nor is the room acoustically treated in any way

Curve is actually a bit smoother at the top than shown due to additional crossover mods I’ve made since. Ignore the "floor/wall bounce depressions" at the low end.

Note this is ~+-5 dBA from ~32 Hz to beyond audibility (with a nice smooth curve) on the couch with no eq or room correction being used! And no woofer foams to ever replace.

http://www.audioheritage.org/photopost/data//500/medium/DSC_001011.JPG
http://www.audioheritage.org/photopost/data//500/medium/DSC_00171.JPG http://www.audioheritage.org/photopost/data//500/medium/DSC_00151.JPG
http://www.audioheritage.org/photopost/data//500/medium/DSC_00122.JPG

Take a listen:
https://youtu.be/JpjCAtrBjTI
https://youtu.be/qzjztUEy2V8

Amps/preamps with tone controls would have come in very useful right now. You could have just turned up the treble and voila. Simple.....

Well, since size of the tweeter is your criterion, then the "best" would be the three panels in the Kingsound King III electrostatic speaker, of course. (reviewed for Dagogo.com) 

Tweeters

An AMT tweeter is like a bellows it’s surface area is 3-4 x it total
dispersion area that is why it too has a wave guide .The Spatial use a huge AMT to cover midrange on up ,where others use a specific driver just for midrange which ion a $$ speaker is more costly like a Accuton driver, which also use a diamond particle tweeter which is $$ but very accurate. The spatial is very good 
upgrading or requesting a Xover upgrade will bring Huge results most everyone skimps In capacitors resistors , Path audio best made period but $30 each , vs $3 ,and many upgrade capacitors available. Inductors like a Jantzen great quality,value not too costly 
but many use cheap bobbin type or called sledge hammer coil of 
wire around a magnet 🧲 you see this often in many under $10 k 
just to save under $100. Just for the record when ordering ask to 
have Xover upgrade options  humble homemade Hifi capacitor test gives a very good comparison for modt caps.
Spatial uses a Beyma AMT made in Spain which is very good 
other companies use a smaller AMT just for upper frequencies like Monitor audio for  example Their pl500 can go up to 100 k which is class leading ,even their studio can go to 60 kHz which is like a super tweeter.