I would imagine an intelligent terrorist seriously contemplating a biological or chemical attack would probably target the water or food supply system.

Poisoning the water supply for a major city could cause real mayhem. Though pretty much anything poisonous enough in large quantities would work (like dumping a few buckets of mercury), imagine using something really toxic that is odorless, colorless and possibly does not affect the victims instantly but only after some time. Meaning that a lot more people will get affected during the incubation period. This is one of the few biological/chemical scenarios that really scares me. More so if you realize how poorly guarded the water supply is in many places.

Option #2 might be less effective, but still pretty bad. Poisoning any kind of food, especially something that comes prepacked, might come fairly close on the Richter scale. Just think of the melamine panic in China not that long ago. Urban dwellers are highly vulnerable to that kind of threat in the Western world.