Karato is one of my family’s favorite onsen. It is a very good generic example of what a supersento is.
I love the rocky shallow spring water filled rotenburo. Overall, the entire bathhouse is one of the best designed and laid out supersento that I've been to. It has everything a good supersento should. I enjoy the dry sauna the most, but the salt one is clean and comfortable too. The two sides are different and change daily. Outside there's some equipment for stretching too.
Given that it's across the bridge from the train station, has a good selection of baths, and the high quality of the spring water Karato no Yu is a great introduction to supersento for a first timer.

Sadly Karato has seen better days. It is looking more and more worn each time I go. I also had an issue with the staff monitoring the behavior of the customers--email me if you need to know. That said, there are toys in the baths for kids and it's a fun place so I can still recommend going.