This old onsen on Kuramaguchi-dōri is Kyoto's best. It boasts an outdoor bath, a sauna, a cypress-wood tub, an electric bath, a herbal bath and a few more for good measure. To get here, head west about 400m on Kuramaguchi-dōri from the Kuramaguchi and Horiikawa intersection. It's on the left, not far past Lawson convenience store. Look for the large rocks.
Be sure to check out the ranma (carved wooden panels) in the changing room. Carved during Japan's invasion of Manchuria, the panels offer insight into the prevailing mindset of that era. (Note the panels do contain some violent imagery, which may disturb some visitors.)