1. Size-The one that fits your hand and allows you to keep perfect trigger finger placement and easily retain the weapon with a normal grip. Controls should be reachable with the shooting hand and with very little repositioning of the firearm.
2. Sights-You can't accurately shoot what you can't precisely aim. Sights must be tall, crisp, and sturdy.
3. Caliber-Standard police calibers are always a good choice: 9mm, .38 special, .357 magnum, .40, .45 ACP are all within good defense range and readily available. Ammunition is more important in distinguishing between these calibers than are the actual calibers themselves. Look for a well regarded JHP around 1,000 feet per second. Slower or faster is fine provided you get proper bullet expansion and have good recoil management. (Recoil management is about the shooter, not the gun.)
4. Comfort-Make certain you handgun feels natural in your hand and is comfortable to shoot.
5. Action-Any modern double-action revolver or modern pistol in single-action, double-action, or one of their many variants.
6. Capacity-Revolvers need 6 rounds and pistols need a minimum of 8 rounds. The new book Practics Handgun explains this further but these minimums work best for self defense.
7. Ambidextrous-Every defensive handgun must be useable with either hand or one broken finger may make you unarmed.
That's about it. At the risk of offending gun nuts, I'll say it plainly--any modern pistol or double action revolver meeting the above requirements is a good choice. It was never about the guns, its always been about a shooter dedicated to successful training and building a knowledge relationship with one defensive firearm. You can't know the traits and performance quirks of a particular gun if you change guns constantly. A .45 and a .357 magnum will require different points of aim at longer distances and will vary in their penetration dangers. So, if you have a good gun--keep it and train. There is no better handgun than the one you can operate proficiently.
I know the above will not satisfy gun lovers, so I'll give two choices. My choices are not patronizing, "good enough for you" choices but rather what I would buy for a self defense handgun. I'll choose two handguns:
1. Model 66 Smith and Wesson w/ 4.25 inch barrel in .357. I'd probably get a good .38+P load and pass by the magnums for in-home defense.
2. Remington 1911 R1 Carry Commander w/ 4.25 inch barrel in .45 ACP, and I'd opt for a mild to moderate load, probably under 1,000 ft. per second.
The above are my preferences but they could just as easily have been something else. The point is the firearm must fit you. The choice is a real one and ought to have nothing to do with the fashion of the moment. For instance, 9mm is the best pistol caliber, in my opinion, because it is the least expensive ammunition among the defensive pistol calibers. All you need is enough to build upon. Not about the gun. Be practical and leave the vain arguments and emotion to the gun nuts: Sig versus Glock. Yeah! Who cares?
Friends, the new Practics book is coming and sign-up for advanced notice is available on this site. The new book is an A-Z training guide for defensive handgun. It is not a marksmanship primer. The Perfect Pistol Shot will remain my best effort at explaining how to get your rounds where you want them.
Thanks for reading.