As a security guy my long bet is transportation infrastructure. Industrial systems (like cars) typically have some very rudimentary security with no depth.

Someone will get root somewhere and shut down some small percent of light vehicles (i.e. not medium and heavy trucks) on the road at an inconvenient time causing a massive economic panic and screwing up the used car market in a way that makes cash for clunkers seem well thought out.

Even a complicated attack vector that results in a narrow target selection (e.g. android malware -> infotainment system on a particular few model years of one brand) would have a massive psychological impact.

Edit: a similar occurrence could happen by accident (edge cases, poor testing and so on).

Edit: meant to reply to a comment. Oh well.