There are 13 free hot spring in Nozawa Onsen and if you only have time for one, this would be my choice. The main reasons? The building is (now) nice as they recently upgraded, the size of the bath is larger than others so it's less intimate, and the water isn't ridiculously hot like a lot of the other onsen in the area.

Also, heads up that while there are 13 "free" baths there is a spot to donate at the entry. These baths are cleaned and maintained by the locals and a small donation of around 100 yen or more is always nice for an incredible experience