I would probably pick a shotgun. It's the only gun that can really fill all roles, even if it isn't that great at any of them.
The N-Strike Recon CS-6.
you beat me to it.AR with multiple uppers
Only one gun for the rest of my life? I guess it would be a Remington 870. I have to be able to hunt as it is in my blood. So for birds, ducks, deer, rabbits, etc, you have to have a shotgun. Make it a nice Wingmaster in 12 g. one barrel with choke tubes and another rifled barrel.