There is no such thing as "best sounding" dome tweeter. It depends on what you want to do re design and what woofer/mid you pair it with and lastly, your taste. I have liked Scan Speak and Hiqphon. They each have several models and can be checked out at Madisound and Zalytron respectively.
Make sure you make a good match with your other driver(s) and a good decision re crossover design. Your question sounds like you want to pick out a tweeter alone as if it were going to play a tune by itself and that approach to driver selection is almost doomed to failure.
Might be good for you to look at the Madisound.com speaker building forum and check out what folks are matching over there. If this is your first diy speaker, and you want real good results(your not doing this as primarily a learning experience) you might consider copying an existing speaker design.
John K, Dave Ellis, Dennis Murphy and others all have well documented good sounding stuff over there.
Also, take a look at the Loudspeaker Driver Selection Guide (LDSG) website at www.snippets.org/ldsg/intro.php3 if you have not done so already.
If you know all this sorry for the lecture.
Good luck
I remain,
