What speaker has the best tweeter?

The one that does not give you listening fatigue & the feel that "it's playing too loud".

Plasma tweeters are physically perfect (you can’t reduce the mass applied to excite air any further) but what has been referred as ‘smell a little’ is their production of ozone.
The LD50 of ozone for humans is around 50 ppm, so this is a real concern and this is why they are not made anymore.

I don’t know if one can evaluate a tweeter alone, , in isolation from other drivers in a system.  There are many fine examples, that can work well in the right system, such as Magneplanar ribbons, pleated ribbons (e.g. AMT), and one of my favorites—Western Electric 597 fieldcoil compression tweeters.  The 597 is particularly good at disappearing—you hardly know it is working.  I heard a fantastic version of the 597 made by G.I.P. Laboratories (this tweeter costs $60,000 a pair); it is in the running for best tweeter.

George - looks like you need a 49 ppm quantum tunneling ozone fuse….

There are a couple of great Beryllium tweeters. D3004/6640, TW29-B, D2908-7140, etc ... You have Raal. And more. Yet for me, I will mirror what others have posted. A good AMT just sounds very real to this 40 yr musician veteran's ears. I love some Aruum Cantus. And although I'm not a fan of Heil dipole tweeters, I love the old round face monopole AMTs! If crossed over high enough in a 3 or 4 way, they are among the best available today.