This question is a case of horses for courses. As a low altitude weapon, the title undoubtedley goes to the FlakVierling. For medium alt, I would say the 40mm Bofors, and for heavy guns, the US 5" piece mounted on BBs, CVs and cruisers. Not only did it contain more fragmentation material than the 88, but by the later stages of the war it was proximity fused, making even a near miss fatal.