Shiobara Onsen Masudaya

Masudaya draws its own hot spring, piped in undiluted, into a single big bath on the top (7th) floor with a view down over the Hoki River. It's split by gender through the evening and again in the morning (the two sides swap), but from 11pm to 6am the door locks from the inside, so whoever gets there first has the whole bath to themselves for free, no reservation needed. It's a small, family-run ryokan rather than a resort, built around that one good bath.