Umeshu (梅酒)
Umeshu (the two Kanji are "plum" and "sake/wine") is a Japanese "wine" prepared by macerating unripe Ume fruits in alcohol and cane sugar.
My personal preference is at best lukewarm towards Sake, (at least to drink by itself, I prefer to have it as part of an actual Japanese dinner) 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 into 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.