If you ask me which Simpson brush a Simpson fan necessarily must own, then my answer is straight and clear: The Duke. It's a legendary brush. I'm sure my dear friend Mark @A.E. Simpson 1919 will confirm.
“Gigantor”
Yaqi Moka 26mm 2 band Badger.
Great brush, big splay/wall of Badger feel, good density, non-shedding, great value. Just a fun brush that I enjoy.