Sake can have the highest natural alcohol content of all beverages in the world - up to 22 %. It is usually brewed to around 20 % but this is then watered down to about 15 to 16 percent to make finer flavors more discernable. Undiluted sake is called Genshu.
There are other liquors like whisky, cognac, vodka, rum and maotai with much higher alcohol by volume but this is because these liquors have been distilled. Before the distillation process the mash for liquors like whisky and brandy has an alcohol content of only 6% to 10%. Therefore sake is the highest naturally fermented beverage as the sake mash can have an alcohol content of up to 22%.

Genshu: 原酒