The fact of the matter is that different shotguns do better at different tasks. Want to kick down people and shoot doors (or dress up like you do and pose in front of the mirror), or are you worried about zombies and home invaders? Benelli M4, Mossy 590A1, or any other tactical shotgun will do you fine.
However, for upland bird hunting, they are going to be way heavy, not swing as well, and be underchoked. They'll be way underchoked for waterfowl, and heavier and slower than optimal for clay games (although they will bust clays).
A twenty-gauge SxS with double triggers and English stock is probably the best thing ever for grouse and bunny hunting as far as I'm concerned. The Brits figured out how to build the ideal game gun over a century ago and I don't think anybody's improved on that since; light, fast, swings and points well... but two rounds isn't enough for the zombie apocalypse, they're probably too light to be comfortable for high-volume clay sports, they don't come with picatinny rails, and a twenty doesn't throw enough lead to be optimal for waterfowling.
Coach gun? Great for Cowboy Action Shooting or shooting driven convenience store clerks; they won't point as well as a game gun and all of the above re game guns also applies.
And so on. If I were going to buy one shotgun to do it all, I'd get a 12-gauge 870 with a vent rib, interchangeable choke barrel in the 26" range and 3" or greater chambers. It's isn't the best at anything, but it will do everything acceptably until you get the urge for more specialized shotguns.
As it stands, I've got an 870 Express for turkey hunting, a tacticool 870 Marine Magnum for range blasting, ####s and giggles, and alarming antis, a twenty-ga. SxS for birds and bunnies, a 12-ga SxS because it matched the 20 and I just plain like English-style doubles, and a .410/.22 over/under that I inherited from my grandmother and hunt with occasionally for sentimental reasons. Each has its advantages.
I'd suggest getting your own place; then you can have all the guns you wan..., er, can afford

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