Either a Rem 870 (my favorite) or a Mossberg shotgun. Get at least 500 shells for it. If you find yourself in a situation where you need to get it out of storage, then you're in a situation where 500 rounds may be what you wind up needing to get you through the next few days
YOU NEED TO PRACTICE WITH IT REGULARLY and clean it after each practice. When things have gone south is NOT the time to be familiarizing yourself with it. It's also NOT the time to discover it has rusted up since the last time you looked at it.
A shotgun (or any firearm) is NOT a magic wand which will visit destruction on any and all bad guys in front of you once you pull the trigger. You DO have to aim the damn thing. More importantly, you have to know how to correctly pump it, and have this be ingrained in your muscle memory, or you will short-stroke it, it will jam, and the bad guy will then insert it up your rectum
Once you've gotten your shotgun, also get a small, reliable, concealable handgun. A Ruger .357 Mag snubby would do nicely. You need it for WHEN your primary gun jams on you at the worst possible time, and also for inconspicuous firepower should you need to go get supplies during a time of unrest and disorder
More importantly, you have to know how to correctly pump it
Personally I prefer a pump but, if you don't plan to practice you might be better off with a Remington 11-87 or if you can afford it a Benelli tactical 12 ga.