In a Survival vein (considering that everything as we know it is gone), probably the Ruger Single Six in .22/.22 magnum. Extremely reliable, ammo could be found almost anywhere (remember you won't have Wal-Mart to go to....), alot folks have .22 ammo loose in car ash trays, junk drawers at home, in the pockets of old or winter jackets, etc. and you can carry a 100 rounds in your jeans.
The important thing would be to be armed. You don't have to have the biggest and baddest gun made.....try to find .40 ammo when that first 50 are gone in a "end game scenario"!!
The same can be said for a 12 gauge coach gun. Simple and easy to operate, not complicated to repair and shells can be found almost anywhere. Remember you may love your .223 black gun, but after the initial "in-your-possession-ammo" runs out - where are you going to find anymore? Oh, I know you are going to say "Well, I know alot guys who have .223's...!!"
Yah, but alot more people have 12 gauge shotguns!!