Steven Levitt advocates a DC sniper approach, multiplied by a number of terrorists acting independently in different areas, but at the same time. While not wanting to take anything away from Levitt - I’ve always thought a concerted Virginia Tech-style attack on a shopping mall would be particularly effective. A small group of well-armed terrorists could cause a huge amount of panic, and economic disruption, particularly if they timed it for maximum impact - say, a few days before Christmas. Heck, they might even be able to get away, in the confusion afterwards.
I highly doubt there’s a factual answer to this question… Wrong forum! 
<mod>
Wrong forum. Bad idea.
Closed.
</mod>