For
everything?
Probably something like one of those Marble Game Getters with a .22 RF or a small .22CF like the Hornet cartridge on top and .410 or, better, 20 ga on the bottom. But with about 20" barrels.
2aHawaii said,
"My one firearm to do everything would be an ak-47 (Converted Saiga). Reliable and flexible and good for everything within 300 yards."
No, not good for small meaty animals... if you want any meat left, and that's the problem with most of the choices in this thread. Sure, you've got magazine capacity, but when the stomach growls, you only need one shot.
For Self-Defence? A .410 1/5 oz slug at 1800 f/s isn't too bad... a 20 ga slug being better. Of course, you only have two shots (upper and lower barrels), but the Bad Guy is also limited to only one gun for everything.* You just gotta practice your reloading.

Just throwing that out there, bearing in mind you can only have one gun for
everything.
And regardless of the fact that ARs are modular, you can bet your sweet bippy that if suchlike legislation occured, you would be limited to only one upper --enforcement problems aside.
Terry, 230RN
* Presuming the BGs will follow the law.... ha.... ha...