It seems like the top 3 choices so far are a .357 revolver, an 870 12-guage, and an AR-15 with multiple uppers/lowers. (Thankfully, I own all 3). I guess I could have added more context in the original post, but that would have narrowed things down and given certain restrictions. But for the sake of continued debate, let's give 2 scenarios:
#1 Scenario: You have to choose 1 single firearm in today's current world. The infrastructure is in place, people go about their daily lives, nothing has changed. Recreational hunting, concealed carry, home defense, target plinking, etc. should all be considerations.
#2 Scenario: Mass chaos. Governments are dissolved, theft and murder are rampant, martial law has come and gone, total disaster. It's "every man, woman, and child for themselves". You have to leave home alone with friends, or with your family to try and get to a safe place and you only have room to take 1 single firearm and some ammo (100 rounds or less). Concealment is not a legal issue, since there is no law, and besides, concealing a firearm might make you and your family a target. Hunting goes from recreation to survival. Also, reliability is a major issue since repair and regular cleaning would not be possible. Also, lots of fancy battery-powered gadgets (lights, night optics, lasers, etc.) wouldn't last too long. One more consideration for this scenario should be accessibility to common ammo, if you come across a sporting goods store or someone's ammo stash.
P.S. - Multiple uppers/lowers/barrels will all count as "one firearm" in scenario #1 where you can store everything, but not in #2, since it's all extra stuff you would have to carry.
For me:
#1 - Remington 870 Marine 12-guage (18.5" barrel for HD, with a 28" barrel for clays and bird hunting, and a 24" rifled barrel for deer)
#2 - A toss up between my good old Winchester 94 lever-action .30-30, or an AR-15 with 16" gas-piston upper in .223/5.56 with flip-up iron sights.
Let the discussion continue...