For a short-term, close-quarters, neighborhood protection environment (like the hurricane scenario described above), I'd think a 12 gauge shotgun is your best option.
That being said, my only experience with something like this was when I was a kid and lived near Jackson, MS. In 1979, the Pearl River, which runs through Jackson, had a 500-year flood. Men in the affected neighborhoods armed themselves (mostly with shotguns, but also with deer rifles and handguns) and banded together to block off the roads that led into their subdivisions. This was done to keep away looters, but also to keep away the curious who might drive through the streets (in the jacked-up pick-up trucks that were popular then) and by doing so create a wake that would send more waves of water into their homes.
If you want to plan for a longer-term SHTF scenario (say, the zombie apocalypse
), you'd probably want to get:
1) a 22LR semi-auto rifle to put food on the table (along with lots of ammo for it since 22LR ammo doesn't show signs of getting any cheaper, it'll last forever of you store it right, AND depending on who you talk to, it'll be THE currency of choice in the post-apocalyptic era
)
2) a high-power rifle to down larger game or zombies (I'd pick .30-06 or .308 since the ammo is readily available).
3) an SKS, because they're fun and your cable or satellite TV will probably be out.