attacking Pakistan would be a very complex problem
Although NATO would certainly have Indian support as permanently removing that problem (from the Indian perspective) has got to be something that they would be in favor of.
The problem however is that Pakistan is a nuclear armed state, and has considerable Chinese support (as Pakistan is a second front against India).
Which is why the US is presently being very careful on how it handles Pakistan even now.
Iran would be the worst of Afghanistan and Iraq. Huge numbers of fanatics (not the whole population, but sizable numbers of them), a better military, a much larger space to have to deal with, and it has considerable support in Palestinian areas and in Lebanon and already supports a terror network aimed at Israel (which could be retasked obviously). On the plus side, the logistics are easier once Bandar Abbas is taken. But very complex nonetheless.
Somalia would require lengthy nation building.. but the military task is easier, fewer troops would be required than either Afghanistan or Pakistan, and logistics are the easiest of all. Kenya would be happy about it, and it would certainly make dealing with Al Quada in Yemen easier. It would also make the task of potentially going into Sudan easier too (Egypt blamed Sudan for harboring terrorists operating in Egypt, Clinton fired a few cruise missiles into Khartoum as well). Of all the choices, it would probably be the easiest sell too. Plus of course there is all of the unsettled business there from the previous US effort to settle things down in Somalia that ended with "Black Hawk Down"
Probably would have been better for the Somali people, their neighbors and the world in general too if NATO had gone in there in 2003 instead what actually occurred.