For me, his prints of abalone divers are the greatest masterpieces in all of Japanese art.
From Wikipedia:
Ama divers have for two thousand years practised their trade of diving for shellfish, seaweed, and pearls off the coasts of Japan. Traditionally they dive naked except for a waistcloth and a cord with one end above the water. They also carry a knife to pry open shells. Ama once numbered many thousands,but by the beginning of the 21st century only about a thousand remained. Though depicted in their youth in ukiyo-e pictures, the ama was a lifetime job that continued into the woman's fifties.
Ama were known for their coarse manners, and their work coarsened their skin. This was in great contrast to the geishas and courtesans who were normally the subject of ukiyo-e art—their manners were refined, they dressed elegantly, they immersed themselves in high art and culture, and they commanded high prices for their services. The ama were perceived as less feminine, but socially and sexually free. Utamaro and others played with these contrasts, portraying ama in elegant, resting poses as they often did with geisha, but nude and unkempt in their work environment. While Utamaro's supple, energetic, and vigorously healthy ama come across in a way erotic, his depictions of them differ from his often sexually explicit shunga. It has been speculated such prints of women who appeared nude in broad daylight may have arisen in reaction to government suppression of shunga.
https://en.wikipedia.org/wiki/Utamaro's_pictures_of_abalone_divers