Umeshu plum wine
Umeshu (the two ideograms are "plum" and "sake/wine") is a Japanese "wine" prepared by macerating unripe "ume" fruits in alcohol and cane sugar.
Personally I dislike Sake (yeah, yeah, I tried it both at room temperature and properly warmed) but I must confess that Umeshu for me was an instant hit.
It is sour and sweet at the same time, and seems to taste great both alone or along with spicy/salty food.
It can be drunk chilled, on the rocks, at room temperature or even warmed up in winter. Japanese restaurants can serve different cocktails based on this.
I was initiated to the pleasures of Umeshu in Hungary, of all places, and I started looking for a source as soon as I came back to my home town.
Luckily it was easier than I thought: any well-stocked asian foods shop seems to have it. The best seems to be produced by Choya but other brands may be easier to locate, including Kikkoman.