Everyone in the gun community has an opinion on this issue. If the western world were to come to a grinding halt, I'd personally want an "assault-style" rifle. I hope it never happens because i like things the way they are here, but if something bad were to happen, I'd like to have guns on hand. There are two guns that I'd want in a doomsday situation: a 9mm handgun as well as an AK rifle.
For 9mm handguns, it wouldn't really matter what, as long as the gun works well and is relatively high capacity. When i say high capacity, I think 15 rounds or more.
I like AK rifles because they don't need any upkeep and they'll always work with whatever garbage cheap ammo you can find to feed through them. The reliability in adverse conditions is unparalelled. Covered in mud, sweat, blood, and more, it'll still work just fine. I want a gun with a near zero failure rate if the shit ever hits the fan.
The other popular assault style weapon in this country is the AR-15 platform. These guns have something going for them: ammo availability. If we were in a situation where the military was deployed stateside to try and wrangle the populous, there would undoubtedly be 5.56mm ammo floating around. Having a self defense firearm that takes the same ammunition as the cops and military have would be a huge benefit in such a situation.