The best tweeter design

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.....

There is no real answer to this question. Asking for the best tweeter without reference to other drivers in the design is like asking what is the best tire without asking what application it would be used in - track, draf racing, off road, wet weather, etc. Like many of you I have heard tweeters utilizing ion, diamond, beryllium, ribbon, silk, paper, aluminum, titanium, etc. I couldn't tell you which is best unless you tell me the type of speaker design it will be implemented in. For example, the Maggie ribbon tweeter, although fast and airy, doesn't integrate well withe planar magnetic driver so I find it difficult to enjoy the music sometimes. However, the ribbon tweeter in an Apogee Diva integrate very well with the midrange ribbon, so in this implentation sound is coherent and example of excellent tweeter implementation. I like prefer the sound of silk domes in most cone designs because they integrate well and sound NATURAL, unlike some ceramic or beryllium tweeters I've heard.

Easy! Plasma tweeters are the best - period. But I am biased :)

Plasma tweeters excite air movement directly, with no mass. The sound is the purest you have ever heard. The only downside is cost.

Diamond cones , ribbons are also fast but directional and dont have the same snap /impact in my opinion .
I would suggest to use diamond cones also for lower freq ,as low as one can go for accurate (freq) response they have very lineair response and very low distortion , but at a cost.
The berylium cones i have not yet had much expirience with