If there is only one gun to have, there are many arguments that will be brought up saying it should be a Glock 19 in 9mm. Why? Because the Glock 1911 is rugged, durable, holds a lot of ammunition, is compact and easily concealable, large enough to fight with, and fires the enormously popular (and currently cheap) 9mm round.
That’s also not to mention that Glock has a superb reputation for reliability, and the Glock 19 is currently the best selling handgun in the United States. Spare parts and accessories and holsters are widely available. In addition, the Glock 19 will also accept longer 17 round and 33 round magazines, further increasing its overall firepower.
In short, a mid-sized 9mm pistol with a large capacity and a relatively compact size to make it easily concealable will be a very valuable sidearm to have with you in a disaster scenario.