My poli sci class says the best weapon to beat terrorists is to pay off the moderates with funds and friendly policies then alienate the extremists. They'll then be more vulnerable when you kill them with conventional means since they'll lose the ability to blend in or receive local support!
Bioweapons are only really useful if planned insanely well. For example, unleashing say...Ebola in a major foreign city would work fine. You wouldn't have to worry about incorrectly IDing someone with the virus, and they could hardly get past airport security. Meanwhile, when the government tries to help the sick, they'll inevitably spread it to a limited degree to nearby areas. Your government won't need to worry about the other country's sick (they will if only for the sake of saving their workforce), and without that responsibility your government could isolate any incoming infected. Ebola kills too quickly and is too obvious to spread more than a couple hundred miles, aside from planes which will be isolated zones of infection since infected on them could be contained due to ease of identification and immobilization of the infected. So, that makes it a fairly safe bioweapon, so long as the target country does not know who did it.
Bird Flu would make a terrible bioweapon, since it takes a few days to infect and doesn't have obvious symptoms. The risk of it spreading to your country is almost an absolute. But, it could be made to look like a natural outbreak.
(Note, these are for state use. If a terrorist wants a bioweapon, smallpox would be a good choice, as would be bird flu. Ebola isn't bad, but the range is too limited. I suppose for the sake of terror it is pretty effective.)
[This message has been edited by Cheesewiz (edited 04-11-2010 @ 04:03 AM).]