Something that will get you through anything and always be there to fire. Something you can always count on, no matter what. Cold, heat, wet, dirt, mud, drops, falls, bites, claws.
Is there anything in your mind, that you trust to take all that and keep on tickin'?
The year is 1997, you’re living in an alternate timeline where mars has been colonized for decades. You’re in Habitat 77, isolated from the rest of the habitats. G-raptors, genetically modified dinosaurs breed to survive on mars prowl the halls. What modern firearm do you choose?
I reject the premise of the question. Different situations call for drastically different equipment:
A summer in the woods in Alaska poaching small game for food: bolt-action .22, maybe a CZ 457.
A winter in the woods in Alaska poaching small game for food: forget it. You're going to die if you're asking questions like this.
A desert island: flare gun.
Urban area, breakdown in social order in which you have gunfights with your neighbors over a rapidly dwindling supply of resources: Lincoln's Repeater.