Best? At least around here, I'd vote for a friend's NW Critter ID -- 4 class sessions for reviewing slides of the critters and then 4 dives where you have to catalog what you see.
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Maybe I'm being unfair - natural suspicion of new age hippy stuff.
There is an interesting list of weird and wondeful specialty courses here:-
Distinctive Specialties | PADI Instructor News
I was also rather amazed by "Frogfish specialist" and "Indo-Pacific Nudibranch Specialty".
But I didn't want to turn this into a PADI-bash. I like the fact that there are really good distinctive specialties that ought to more mainstream.
...... - Underwater model (sheer genius!) ......
AndyNZ has a specialty titled something like "Self-Reliant Diver" that I think would be great.
What about really good ones? I thought twin-set diver was a brilliant idea. Can't believe PADI haven't copied that generally.
i will add one that I have... "Aquarium Diver" the only upshot was the back end fish ID and education that came along with it.