What is the most devastating single terrorist attack in history?

[sirio11]

Dictionary.com define a terrorist attack as "the calculated use of violence (or threat of violence) against civilians in order to attain goals that are political or religious or ideological in nature."

Some of the other threads make me think about this. The Guiness book list 9-11, but I think the #1 should be the bombing of Hiroshima (140,000 civilians killed) and #2 the bombing of Nagasaki (70,000 civilians killed).

What is your take on this?