The best tweeter design

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

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

Don't forget the health hazards too. :D

Face; "a tweeter to die for"?

The new plasma tweeters have catalysts that convert Ozone back to Oxygen - so no need for Helium tanks or gas masks :)