We want a cartridge that has sufficient stopping power or knock down power to stop a threat! What does that mean? Well, things have changed a lot since I started carrying thirty years ago! Just in the past few years ammo design and technology have changed the game! Any high performance hollow point ammo will usually stop a person from doing what they were doing! .380, 9mm, .40 S&W, .357 Sig, .45 ACP, are effective man stoppers. Obviously, some are more effective than others, but all are capable of stopping a threat and saving your life!
Shot placement is very important. Practice, practice, practice! Find a shooting venue where you can draw & fire and you can also move and shoot. You need to practice what you will actually have to do in a self-defense situation!
Let's talk specifics. People who are recoil sensitive, women and older people with arthritis will want to consider the .380 or 9mm. Everyone else can shoot whatever they prefer. An ER doctor once told me the .40 S&W was the most devastating round in his opinion. So a .40 is a good choice! It will have a very sharp recoil in a smaller and lighter carry gun. The .45 ACP cartridge is larger and heavier and a little slower bullet than the .40S&W. It too is an effective self-defense round. The 357 Sig is also a great choice but has a few draw backs. It has a fair amount of recoil and it can be over penetrating at close range. It is also rather expensive to shoot and there are not a lot of hand guns chambered for it. For you revolver people, the .38 can be a good man stopper, however, I would not recommend using the .357 magnum in the small light weight revolvers with a short barrel. The recoil and muzzle flash will be hard to overcome!
Here is the bottom line. Don' get paralyzed by over analysis! Like I said earlier, any modern high performance round can save your life! Another point, people who want to shoot a lot and are price sensitive will often want to choose the 9mm. It is a good man stopper and is very affordable to shoot! The most important thing is to carry all the time…period! Whatever you decide to carry is worthless if it's not on your person and is in the car!! So, the most effective round is the one you brought to the unexpected gun fight! Practice like you may need it! Pray you never will!