Well, a diamond is a Carbon, not a metal.
Thus, beryllium may arguably be considered the best Metal tweeter.
But, as well all know, it's not just the drivers that make the speakers. It is the implementation.
For example, the RAAL Aluminum or the Coincident Titanium.
Diamond may be considered the best hard dome tweeter by B&W. It is certainly the most expensive. The cheapest aftermarket diamond tweeter is $2,000 each. A pair of diamond tweeter speakers would cost $4,000 for just the tweeters alone.
Dang, the B&W 805D2 seems like a bargain at only $4,000 delivered from some dealers ($5K MSRP).
But others from Revel, Focal, Paradigm, RBH, etc, believe that Be is the best driver period.