Biological and nukes for my votes.
I'm looking at a map of hub airports now. Whether it's a new Ebola or zombie variation modern transportation will be vital in spreading it. "Shamblers" were done by T2013's Halloween special and are in another thread. I guess I'll try "Ragers". BTW, "Shamblers" are traditional walking dead, "Ragers" are living infected. It would take some effort but the zombies could be translated over to T2K2.2.
I put off polling to make sure I could spark good ideas, I can be socially clumsy. Twilight 2000 can support quite a few scenarios and I wish that GDW had started their attempt earlier.