I can't state one drama/film :p
Best Japanese dramas: Hana Kimi, Liar Game, Kurosagi, Yukan club, Mr. Brain and Life.
Love Shuffle, Hana Yori Dango, Pride and Buzzer Beat when it comes to romantic Japanese dramas.
Best Japanese films: You have to take in consideration that I adore Japanese horror films so my favorites will be: One missed call, the grudge, dark water, the ring, the curse and so on.
Other than horror, there's Confessions, Kimi Ni Todoke, Mother, GO, Drop.