The best tweeter design

I've been speaker building for 32 years. The best tweeter that I've ever heard is the old Hill Plasmatronics.
I've had great success with ribbons, but they can be beamy and one that can cross low is expensive.
Horn loads: (you can hear the horn)
Diamond dome:(Accuton) Haven't tried them... I'm always on a budget
Ring Radiator: (old advents) ok
Inverted Dome: (Focal) Fast, Detailed, can sound etched & grainy easily with some electronics, wonderful with others.
Soft Domes: I currently use a Scan Speak with excellent results. Alot of variations with very different outcomes.
Bottom Line, use a technology correctly and you can come up with a very nice sounding speaker.

As far as ribbons being beamy. In my experience, not all ribbons are beamy. It does have more to do with ribbon design (dispersion pattern) but not all ribbons are beamy. Also, some of this can be overcome, by crossover frequency & slopes. I've seen open back and sealed back ribbons and one where the ribbon was mounted horizontally. Hopefully everyone has sat in front of a good set of ribbons, what they do, they do very well. Again, I've always been a budget builder. I've tried a few sets of HiVi's, theirs are a mixed bag and some of their ribbons have an extensive rise on the high end that bothers me, One in particular was quite good. I've played with Bolender Graebner and an old Fostex. Some of the Fountek look promising and I've seen great results with RAAL. Unfortunately, I have not yet figured out how to just look at a ribbon and tell if it will be beamy. I know none of this is really a help, but to say, there is alot out there, some of it is very good.