second kevins comments on this matter. There is no perfect tweeter and also no single value which gives us a clue wether the tweeter is good or not. All kinds of things have to be factored in. THD, freq response, power handling... it is all accumulating to form an overall big picture. Depending on your design it can even be the size of the faceplate that has to match the look of the other drivers and whatnot.
There is no such thing as a single master measurement or value by which you can judge drivers.
Also ones experience with a certain driver can be worth mentioning. If you built several deisgns with the XYZ then it might be a good idea to call upon that experience since it will definetly make things a lot easier.
If i had to put it in a nutshell:
freq response
thd+ spectrum
power handling also including similar efficiency when compared to the other drivers of the speaker
Also id like to mention that i, too, do not believe in the idea of slow and fast tweeters. Althouh some tend to use this term a lot there is no such thing as a slow or fast tweeter. The speed of sound is always the same

A tweeter in itself is never slow. Combined with your xover you might be able to see differences in rising time/ impusle response and whatnot when measuring the speaker.
Usually thewoofers will have like 25 to 35 ms delay and stuff. If you have an active speaker you can easily play a bit with it. Why not try building a closed baffle woofer which has pretty good and fast response as we tend to say.. and then simply use the dsp xover to set delay to 20ms making the fast cb woofer slow intentionally. You will find out that what is often described as fast bass is not really the delay but instead its simply the fact that cb sounds differently than br designs...
just my 2 cents