Remington 870/Mossberg 500 - Atrociously reliable, multipurpose (hunting/defense/trap), 12ga ammo is cheap and common. I personally think clay shooting is one of the most fun shooting sports and a great way to get other people into shooting.
.30-06 bolt-action rifle (Remington 700, savage 110, etc) - Scope, this big enough to hunt any North American big game.
AR-15 derivative - Defense and just plain fun, not to mention that this is a great apocalypse gun to own.
1911 pistol - So much fun, accurate, use it for competing, infinitely customizable.
Plastic pistol (Glock or xD) in 9mm - Cheaper shooting, better for taking friends shooting than a .45, I'll probably die before it does.
.357 or .44 magnum revolver - Carry while outdoors, also a gun that's great to show around.
10/22 - I'm not sure this needs a reason, I think most gun enthusiasts know about how much cheap fun these are.
AK or derivative - Bigger caliber could be more useful than .223, ubiquitous parts and ammunition, so cheap you have to own one.
.22 Caliber revolver or pistol (or a conversion kit for the 1911) - Cheap plinking fun and the cheapest way to build handgun skills.
.50bmg rifle - Because...well, if s*** hits the fan and you have to defend yourself, this is a pretty good way of reaching out and stopping cars or trucks, or just reaching out 2/3 of a mile and touching someone. Did I mention that you can make 1000yd shots with it
?
And I only own a third of them...yet. I think that after boring stuff like apartments, and cars, and that malarky, a .50bmg or a real AR might be the first thing I buy once I move.