I'd suggest that your SHTF rifle needs may vary depending on where you live.
Unless you plan on sitting out Armageddon in town (probably a bad idea), your needs are going to be zombie defence (in the comic allegory for other more realistic but less than politically correct or socially acceptable threats sense) and food procurement. If you're going to stay in town, anything that'll work on zombies would do.
Assuming that you're going to be sitting out the crisis in your rural, well-stocked luxury survivalist bunker, I propose the following (and make no claims for originality):
A .22 is arguably a must have. Light, effective on small game and zombies (with careful shot placement making up for the low stopping power), ammo is plentiful and cheap and you can lug a couple of thousand rounds around all day long. And reportedly, you can rig homebrew suppressors for them, although that would be highly illegal. A 10/22 with a bunch of BC 25-round magazines makes great sense here;as somebody pointed out a 25-round burst should do nicely on a zombie.
I'd say that a 12-gauge pump is also a must have. Ammo is plentiful, the gun is incredibly versatile and can take everything from bunnies to bears, and is arguably the ideal weapon for close-range or in-bunker zombie defence. As somebody else pointed out, an 870 Marine Magnum is tough to beat, and I agree (I named mine "Alice").
For longer ranges, you'd need a rifle. If you're looking for a pure zombie gun, any .223 black rifle would do just fine, but I'd argue that as you may be using it for hunting as well as defending your bunker from the zombie hordes, you might want a more powerful chambering. I'd suggest that anything that meets the following criteria will do:
Reliability: you want something simple that'll handle a lot of abuse and neglect and still fire. The AK-47 design philosophy applies here; looseish tolerances aren't a bad thing if the rifle will keep on firing reasonably accurately while filthy. The zombocalypse isn't the Olympics; you don't really need minute of angle accuracy; minute of zombie or minute of deer torso accuracy will suffice.
Power: you want something that'll kill any animal on the continent, from polar bears and moose on down. I'd argue that you want .303 as a bare minimum; as others have said, it's a proven moose-and-Nazi killer.
Cheap, plentiful ammo: for obvious reasons. You can loot as many burned-out, abandoned Canadian Tires as you like, you still aren't going to find any .416 Rigby. .303, .308, .30-06, on the other hand, you might.
Portability: you may be lugging it around a lot. With ammo. Something not too heavy or too long will be a lot easier to lug.
High-capacity magazines: because zombies frequently travel in shambling, putrefying hordes.
Rapid-fire capability: because zombies frequently travel in shambling, putrefying hordes.
Back-up iron sights: because scopes break, and if your local Canadian Tire or gun store has already been looted or you haven't got a spare, a rifle without sights is useless.
Sturdy, low-maintenance construction: this should be obvious.
Lee-Enfields fit the above (as would any of the 20th-century military bolt guns, subject to ammo availability). A good SKS would too, although they're a bit short in the power department. Any of the lever guns in an appropriate caliber would do the job. Norc M-14's, any of the fine hunting rifles suggested by the other posters in this thread... really anything that fits the above criteria and works for you.
), and in the Arctic (now, 
), the one firearm I would consider in both environments is a 12g pump with extended mag. It is a fabulous short-range and a fairly decent mid-range weapon. It would put small and big game on the table, and it would strike fear in most zombie hearts. I'd be up sh*t's creek if I was dealing with a zombie sniper though! 
