The best tweeter design

That High Emotion unit sure looks interesting. I'd love to give those a listen. Please correct me if I'm wrong, but, aren't compression drivers limited in power handling?

The High Emotion tweeter sure looks like the now defunct Linaeum.

There is no best. Only best choice for design. But of all the hi frequency transducers Ive heard used and this incs $5k diamond domes, plasma and the better ribbons was fostex t500amk2 but then only with matching horns.

I can see that you already concluded that Diamond dome has to be superior to Beryllium dome. Steve Mowry says in the article "The Whole Truth About Beryllium Diaphragms: -

"For a given geometry the first bending (break-up) frequency is proportional to the material Speed Of Sound, where the speed of sound within a material is defined as the square root of the Young’s Modulus divided by the Mass Density, (m/s); the higher the better. However, the Mass of the diaphragm must also be considered. Then the ratio of the Speed Of Sound to the Mass Density can be used as the materials’ acoustic figure of merit, (m4/kg/s); the higher the better."

After that he shows values of "Acoustic Figure of Merit" for different material (higher the better)in m4/kg/s

Berillum - 6.97
Diamond - 4.92
Aluminum - 1.86
Titanium - 1.13
Steel - 0.63

It is hard to beat a Ionovac plasma tweeter, and a horn at that. But, they need to be rebuilt, and are expensive. I also like the Magnepan tweeter/Ribbons, very detailed and natural, big open space, the Legacy Heil tweeters are wonderful, (Bill Duddleston reportedly has every significant tweeter made, and he chose these...I would trust his ears) and the Aerial tweeters on the 20T are wonderful. Domes just donn't open up for me as well. But I do like the top end of the Cremona M by Sonus Faber. Domes can do things like efficency, but many hurt my ears. Personal preference I guess.....