As french, I've seen so much japanese anime since childhood and beginning of the 80s.
But if only one would remain in my mind... It would be not an old one...
I would say Fruits Basket rules!
(but the list would contain City Hunter
, Kimagure Orange Road and Maison Ikkoku as well... and more modern, Kanojo to Kareshi, even unfinished, was a nice thing lol!)