What speaker has the best tweeter?

The best speaker I have heard so far is the Lansche 8.2. It uses the Corona Plasma Tweeter, that George referred to earlier. 

After Lansche's  Corona Tweeter, RAAL 70-10 ribbon and ELAC 4 Pi Omnidirectional are at the top of my list.

Corona Plasma measures up to 150kHz
RAAL 70-10 Ribbon measure up to 100kHz
ELAC  4 Pi measures up to 50kHz - I use in my system.

I have never had the opportunity to hear the Magnat. From what I have read they measure in over 50kHz. 

Accapella Audio Arts uses the Ion Plasma Tweeter, and sells stand alone units which both measure in at around 50kHz.

Plasma tweeters are still being produced. 

Usher DMD Diamond Tweeter.

The best tweeter to my ears that I've heard are those made by ATC; used in their pricier speakers.

I have not heard most of the ones mentioned above, so they may very well be better.

Tannoy pepper pot waveguide, as part of its concentric driver. 

Scanspeak Ultra tweeter.

Made exclusively of silk from free-range silkworms fed on organic, non-GMO mulberry leaves.