Yep Rip, I fear biological weapons more than anything else.
Imagine an "improved" genetically manipulated Ebola virus, in aerosol form. The ebola virus has an initial death rate of around 85%, diminishing with time, but it could probably be manipulated.
Spread in one airport, it'd quickly spread around the world, and only the 0.1% of the population that have a genetic setup to combat the virus would survive. If they're exposed they risk death, that is.
At any rate, the dangers of biological weapons are, due to the inability to control them once spread, far greater than a rogue state with a handful of nukes.