The main thing you need to consider in a SHTF scenario is what will be most readily available.
As for sidearms, I would have to go with either a Beretta 92/M9 or a Glock of some sort (even though I fricking despise them). If shite hits the fan, military supplies and police department supplies will be most readily available. I love 1911s, M&Ps, XDs, etc. but those just aren't as common. Even as anti-Glock as I am, I can see the up sides to having one. I would definitely go Beretta 92 before Glock though. 9mm would be the easiest caliber to obtain as well.
For combat rifles, AR15 all the way - Same concept applies. Extensive use of the AR15 in military and PD applications makes it the most common. There would be parts and ammo everywhere for it.
For a precision rifle, the Remington model 700 also follows the same line of being used in military and police applications. A 700 in .308 would do the job just fine.
For shotguns, the classic Remington 870 or Mossberg 500 are about as common as they get. For close quarters, a shotgun is ideal.
And for a .22, the Ruger 10/22 is definitely the most common.
This post was edited on 10/7 at 4:15 pm