What speaker has the best tweeter?

No dynamic tweeter not even an Accuton Diamond tweeter can match the Magneplanar ribbon. 

Mozartfan, those specs only tell you how you can use the tweeter. They tell you absolutely nothing about how it sounds and works in a room. The problem with most tweeters is that they spray high end all over the place worsening room interaction. This plays havoc with correct imaging and tends to increase sibilance. The Magneplanar ribbon and ESLs do not do this and even if you boost the treble sibilance is absent. Dipoles do not send any energy to the sides minimizing reflections off of side walls, line sources have the added advantage of not radiating either up or down.
Next time you are in the presence of a dipole walk around it and see what happens.

Maggies all day.

I just spent 2 weeks without my Gallo Reference 3.1s (blew a midrange driver in both speakers). Drivers are now replaced with new old stock drivers from a Reference AV center channel. I can’t think of a better tweeter than the CDT tweeter in the 3.1s (and other Gallo speakers). Going from my back-up speakers (ProAc Response 3.8s with Scanspeak soft dome tweeters) back to the Gallos was astonishing.  Like going from listening to boxes to listening to real music. Don’t know how else to say it. The image just hangs in mid air, and you can almost feel the mid to high frequencies in your chest and head (like headphones).  Which makes no sense since its not a bass heavy speaker at all. Just addictive.