What speaker has the best high frequencies?

I has to do with what you like. I’ve used AMT. Some are very well behaved, some can drive you nuts. The reason for the lower XO point, some can go pretty low, for tweet duty. The problem is the higher range they can be tough to manage..

I Normally used them as a mid more than a tweeter. 300-12 khz or so.
Then I use a true ribbon and/or a planar, depends..12 khz up..

No hard domes for me. A lot of the manufactures use them.. they suck.. simple..
I don't care who makes them...

Aurum Cantus is one of the best tweets top to bottom...Either Striction or Ribbon

I try to only use round speakers from 300 hz and down. It makes for a
VERY fast system...Ribbons, planars and domes, are all faster than any round/cone style speaker...

Regards

ESS was a good one too. Actually a great topper for the older boxes
Didn’t JBL used something like that too. Not real savy on JBLs.
AMT an older JBLs? 45 years ago, or more..

I know, I like the sound of JBL. Just I couldn’t afford what I really liked.

+ 1 more for Mrs Jennifer Warnes can we add a few more? 

Regards

 Magnepan 3.7 WOW I forgot about that speaker. That is a really good speaker.  I heard the 3.7i with Mac C50 and SS MC601s (I think). He was selling the C50. I was really impressed.  Did't buy the C50, sure was tempted to make an offer on the MG 3.7i though..

We listened to Strunz & Farah "Rainmaker". WOW! Top 5 best production speakers I've ever heard, excellent indeed.

Regards