Part of it would depend on your definition of "worst" (not worse, for that matter). In volume, it's easy- the music industry. The files are small and plentiful.
But I look at harm to the industry, in which I have to say none of the above, really. My answer is 'niche markets'. Of the 4 listed, video games are the most niche, so I vote for that. But I would say that piracy of things like, anime, are worse. As someone who is an anime fan, I've watched piracy kill the industry, both abroad and in Japan. (Japan's exports to eastern Asia are down 90% since online "fansubs" took off.) Other niche markets also are worse than games, but so many of the "fans" don't care; they think they're entitled to what they want for free. The category will never go away, they think. Well, it will, if it's not economically viable. And those theives (I see it as theft of intellectual property) are only going to kill it faster...
-dunno001
-On a quest for the truly perfect game; I don't think it exists...
