Start talking about what handgun is best for a backup or hideout and chances are a fight will break out. People's tastes vary tremendously, which accounts for the wide array of great guns on the market today. I've carried a number of hideout guns over the years as a lawman, both automatics and revolvers. Some of my favorite firearms are "little" guns, again, both revolvers and autos. But which one is best? It all depends on personal taste, in my opinion.
My first backup gun was a Walther PPK/S .380 automatic which my dad gave me when I graduated from the New Mexico State Police Academy way back when. I generally carried it when working at night. I tucked the little auto in my Sam Brown belt and holster rig inside the belt, in front. Since the NMSP uniform is black, as is the belt and holster, the little black Walther blended in well. It was also comfortable.
As a plain clothes investigator, I've carried several snubnose revolver models over the years, including the Colt Detective Special, the Smith & Wesson Chief's Special .38, and the Smith & Wesson 640 in .357. While the 640 was obviously the most potent of the bunch, my all time favorite is the Chief's Special, and that's just personal taste. My Chief's Special has the hammer bobbed off, a nickel finish and a great trigger job - plus it was my dad's when he was the Sheriff of Deaf Smith County, Texas.
There are some great hideout guns available today, including a slew of great autos such as the Kimber Solo 9mm. If you're looking for a good backup gun, don't discount a good wheelgun, either.