I think the most efficient attack would be to release an airborne virus... not with the intent of killing people as such, but to incapacitate the nation ... War used to be about killing the enemy troops, until they found out that a wounded soldier costs the enemy far more resources and problems than a corpse.
The modern day terrorist knows that it cannot win a war with a highly funded, highly trained military... their only option is to incapacitate the supply chain feeding the beast...
release airborne and highly contagious agents all over the country and disable the population either through illness or quarantine .... if nobody can go to work, no money is earned and lots of revenue is lost ... that can seriously injure a nations economy...
Now all of the above is speculation on my part, I am in no way planning an attack or trying to bring anyone to ideas
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But my agent of choice would be Guinness
get the population drunk for the larger part of the day ... and see the economy crumble