This in my experiance has been a faulty logic thing. Some people tend to gravitate toward the exotic stuff, and completely forget that way back in the 70's KEF invented a linear, accurate tweeter that was matched +/- .5db. Not to mention ribbons which have been around forever. Many of the top-line speakers through the 70's and 80's used the Kef tweeter. And yes, it was that good then, and it is now...it is excellent PERIOD.

Fast forward until now. We have beryllium, Titanium, diamond (which is actually carbon crystal) ceramic, horn and so on.

Implementation is the most crucial, not the material. How the tweeter is mated with a midrange can make or break a speaker. two good cases:

The Avalon Eidolon diamond uses that mega-spendy "diamond tweet." After listening, I prefer the silky-smooth ceramic version, which costs much less.

Vandersteen 5 uses a slightly modded Vifa aluminum dome tweeter that goes for maybe $40 a pair. These speakers sound awesome! And ATC monitors use a $30 a pair vifa soft dome. The ATCs are extremely linear, musical, transparent and unblinkingly detailed. All this from one of the least expensive tweeters. Km acoustics makes an all-metal speaker using metal woofers, and a focal tioxid tweeter. Yet these speakers are very neutral---with no metallic ring , color or bite. That is great engineering!

The most recent "tweeter" I listened to was the new JM labs Focal Be on the mini utopias. This is not only a superior tweeter to the focal tioxid IMO, it mates better with the woofer in the Jm labs speaker. It's amazing!

So while people have "favorite" tweeter, I feel that I have "favorite" implementations of tweeters reguardless of material.