What speaker has the best tweeter?

Well then since you're not looking at buy/add to your current speakers check out Dali Epicon Series approach use Two!

Twin tweeters are something of a Dali speciality and comprise a fairly conventional dome working together with a ribbon unitbest of both worlds approach The ribbon tweeter simply takes over where the conventional dome starts to tail off, which in this case is around 15kHz. The idea is to maintain treble dispersion at the highest frequency and, obviously, take advantage of the quality of the ribbon unit.

Result - very unique detail sound

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.