I think the options for choice of weapon is limited.

A nuclear weapon would cause retaliation in kind--massive retaliation. (I think at a ratio of 10 to 1 and the strikes would include capital cities, seats of government, military bases and manufacturing centers. I say 10 to 1 because that's what I'd do. But then that's just me.)

A gas attack in different areas of the US in an attempt to overuse and collapse the medical response system and trap Americans in their houses or places of work would engender a retaliation similar to that above or massive air bombardments using conventional weapons but on a scale never seen before.

Terrorists think in terms of crippling political processes and populations. Damage is easy enough to inflict when you get to choose the time, place and method. Once that type of battle is joined, then the military gloves come off, and despite what folks think of Muslim extremists' willingness to meet Allah, the leaders don't want to any sooner than they have to.

Subsequently, I would think that multiple targets using conventional explosives on large targets would be the likeliest scenario. Large oil plants, water supplies and transportation facilities. That attack will be meant to get at America's economic system and 'disallow' the US to respond with its own WMDs. The stock market was closed for about one week after 9/11. The drop in America's 'worth' was about seven percent. China picked up the slack. The USA cannot afford too much more of that. I think that's what they will go after. IMO.