What speaker has the best high frequencies?

Lansche or Acappela with ion tweeters give the best extension and clean treble.

Vintage Western ion tweeter sounds even better to me ( I have a friend who own them) but they are almost unobtainable.

Thomas

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

Piega Coax 711

I've not heard it, but if that's the Beyma AMT I've read from many a DIY'er it is among the best performing tweeters in the world.  Great dynamic range without compression, very low distortion, and the horn adds controlled dispersion, guaranteeing a clarity at the listening location you are unlikely to find anywhere else.

While not AMT's are created equal, some manufacturers use really awful versions, they all share in low stored energy, the equal or better of Be or diamond tweeters in this specific regard.

B&W diamond tweets are not nearly as natural.  If you've ever walked into a store and said "Oh, I hear a B&W speaker!" you know what I mean.

The usual people posting their usual dribble!