If you want to experience not only Japanese food but also a Japanese moment, then Yuki is the place. An open kitchen, a japanese indoor decoration (meaning rare but elegant items of attention like pebbles in the sink), ceramic tableware and traditionally-suited waiters. Kon'nitchiwa and go for the meals. Beside the classic ramen and gyozas (all extremely tasty), you can also taste a few specialties and specials of the day. I particularly enjoyed the Kinpira. They provide a decent choice of sake served either cold or hot and their mochis are top notch. So have a bento for lunch or explore the menu for dinner. Best way might be to jump in blindly.
Strada Puțul lui Zamfir 5, București, Romania