What is the strongest brandy in the world?

Probably "Calvados", an apple brandy. Commercial Calvados is not that strong, but the calvados made in the countryside by French farmers is commonly distilled out at 140 proof.

After the liberation of France in World War II, the GI's were offered Calvados by the grateful French people. GI's, thinking it no stronger than any other brandy, chugged it - and a lot of problems arose from that, as you can well imagine. The saying among the GI's was that it was made from "ground up grenades".