Which was the most effective destroyer: USS Johnston or HMS Campbeltown?
Short answerProbably USS Johnston, because Campbeltown's greatest effect came from serving as a bomb, with Eskimo and Javelin also worth mentioning.
NotesThe choice depends on what counts as effectiveness. HMS Campbeltown belongs in a separate category because her decisive use at Saint-Nazaire was principally as an explosive charge rather than as a fighting destroyer. USS Johnston is therefore the preferred comparison. HMS Eskimo at Narvik and HMS Javelin also deserve honorable mention for their operational records.