Saikyo miso is a traditional white miso from Kyoto, Japan. The color is pale and it has a natural umami-rich sweet flavor due to the higher content of koji rice (malted rice) in the miso. The sodium level is also much lower than other types of miso, so you can use a good amount of it to enjoy its rich flavor in a soup, etc. without making the dish too salty. This hot pot dish has the combination of dashi, soymilk, and Saikyo miso in the broth, so the flavor is rich and complex. Make sure to use the real Saikyo Miso from Kyoto and pure rich soymilk (with no additives) such as Banrai Soymilk.
Buttery fish like cod is perfect for this broth, other types of seafood or even chicken would certainly work in this dish, too.