"Best" is a subjective term.
What is "best" to one person, may not be to someone else.
Kris "Texas" Pfen...
I would suggest that YOU, yourself, decide what things about a howler would make it "the best" to you.
Things such as:
1) What material do you want it made from?
2) Do you want a production call, or a custom made one?
3) If a custom, do you want the toneboard to be made directly from the call body; or do you want a separate toneboard (either hand-made from another material; or a pre-made machined/or molded toneboard?
4) How wide/deep of an air channel do you prefer in the toneboard? (This will affect the amount of air it takes you to use the call, as well as possibly affecting the tonal quality.)
5) Do you want the reed/toneboard to have any tonal/pitch breaks, or none at all?
6) What general pitch do you want mid-reed, and do you want the tonal quality to be "clean & crisp"/"raspy"/or something in-between?
7) Since diaphrams have been mentioned, is that what you want instead?
Once you decide things like this, & can then give that info to the call maker (if getting a custom), the better call the call maker can create for you.