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.

I guess a lot of people have not heard big Mageplanars. The ribbon tweeter in the 20.7 is handily the best tweeter made and I do not expect that to change soon. It does everything right. It is effortlessly smooth and disperses perfectly. As a line source dipole it limits room interaction and will sound great in just about any room you put it in. The only downside is it can be a bit fragile but Magneplanar has an excellent tweeter replacement program. They will have a new tweeter to you in three days. You put the old tweeter in the shipping tube the new one came in, apply the return shipping label they give you and they will refund the core charge when the get it back. The magnet structure is immortal, they just keep rebuilding them.