Best defense is always M2M. However, if i am 100% sure,and let me say that again, 100% sure that the opposition will go with an outside offensive then I would definitely look into using a 3-2 zone. Problems with the 3-2 with the NT is that there are many teams that can run multiple offenses so it is hard to predict an automatic 3-2. You dont want to call a 3-2 only to find out you get creamed with a LI and lose by 30.
As for the 2-3 there has been some great research done by wozzvt and it has shown that with SB and the right amount of OD/ID at the SF and PF positions the 2-3 can be pulled of successfully. The problem there though is that it requires just the right skills to pull it off and we may not have the resources (players/callups/GS) to really play it.
1-3-1.....Im just gonna pass this one because if a 3-2 is hard to run why would we run a 1-3-1 lol
Box and ones are tricky. As for my own team I have had success with it but Im not sure we are at the right time to try to pull it off during a NT game. It may be a possible option some time during a scrimmage to really see how it works but until there is more info on it and more NT matches played with it I dont feel comfortable running them at this time.
So, M2M would be the primary defense and there would be a possibility of an occasional 3-2 or 2-3 but no promises there.