Kenkyusha's Koujien, the "Websters" of Japan.
Discussion:
The long-standing standard reference for modern Japanese. Generally, considered to be the best Japanese-to-Japanese one-volume dictionary, although some prefer Sanseido's Daijirin. My editions of the Koujien go back to the second edition which came out in 1945, thirteen years before my first visit to Japan. Either this or the Koujien is indispensible for working with Japanese.