Okay, I'll play. I'm going to limit myself to what I have in my personal arsenal. Please note that that does not include a .22 right now.
-Lost in the woods
Depending on why I was out there I'd have a rifle (most likely my garand or my mosin) and probably my xd9 or my 1911.
-EMP away from home (have to get back home)
I'd have the gun I left the house with, which would be a handgun in 9mm or the .45, often both since I do like to stick one or the other the center console.
-Societal collapse away from home
Again the 9mm or the .45.
-Societal collapse at home
at my house I'm probably going to go for the 12 gauge.
-Urban evacuation/bugging out of the city-Bugging out to the wilderness
everything I can realistically carry LOL the 9mm and the .45 on my belt, and I'd disassemble the long guns as much as possible and fit them in the backpack, most likely I'd take the shotgun and the garand as both are common calibers.
-Bugging in an urban environment
If i'm bugging in I don't really need to worry about selecting which guns to have on hand, because they're all on hand and the reloading equipment is all right here too. Most likely the ones closest to my person would be the 9mm's and the shotgun.
-Bugging in a rural environment
I would be sure to have my garand, the shotgun, and the 9mm and .45. All very common rounds and I think they'll suit most purposes except small game.
-Home invasion
Shotgun or one of the handguns. Depends on where I am in the house when they break in.
-Car breaks down
I'd be limited to the guns I left the house with, which would be the 9mm or the .45, possibly both.
-Zombie apocalypse
The m-1 carbine. It has a 30 round magazine, I can reload oodles of rounds, it doesn't kick my butt when I fire it, and I hope to scope it this fall.
-Biological epidemic
Hopefully they'd be too ill to be much of a physical threat, but if I want to be really hard I could sit on the roof and peg people out in the street with the m-1 like I would zombies and build up a ring of bodies outside my fence. One would hope that would be a deterrent on it's own. If they got into my house I would probably not use the shotgun on an infectious person. I would want to minimize the exposure to body fluids, not blow a 6" hole in them and subsequently have to scrub and quarantine the room that the 6 inches of tissue splattered. I might try to work up a low velocity round for the 9mm that hopefully would stay inside the person, but if they didn't want to cooperate with a firearm in their face most likely I'd go with a low contact method to confine them like roping them and then get them outside where they could be dealt with.
I do want to say that would be for strangers, for people I know I have two sheds in the back yard that I'd be willing to use as a quarantine/hospital ward.