There are a lot of opinions about regarding what the "best" concealed carry gun is, what caliber it should be and how it should be carried. Treat these opinions as something to inspire thought and not an edict. The type of gun, caliber and carry position are all very personal things. I'd like to offer some thoughts on the subject.
This post will be much like another post I've done, Hierarchy of Firearm Choices for Prepared Families. In that I lay out a pyramid of choices that a person can look at. The most important attributes are listed first, these are the basis that other choices build upon.
1. First, carrying a gun is a better choice than not carrying a gun. This may seem a mundane first step until we look into it deeply. The best gun ever you have isn't helpful as a defense weapon if you don't have it with you when it is needed. Therefore choosing a carry gun (or multiple guns) that you can incorporate into your outfit as often as possible is ideal. The more often you carry a gun, the more likely you'll have it on the unfortunate day when you may need it.
2. That gun must be carried safely. If you don't have a gun that can be carried safely you are creating a hazardous situation. If you don't carry your gun in a holster, you are creating a hazardous situation. Sticking a gun without a manual safety (outside of the trigger) into your waistband is a great way to shoot yourself with a negligent discharge (ND). Keeping a gun in your pocket without a holster increases the risk involved with carrying a gun, especially if you carry anything else in that pocket. You don't want to carry a gun that you aren't comfortable with the safety mechanisms or carry a gun that does not reside in a holster.
3. Next up is selecting a gun that you can shoot well. It's somewhat pointless to carry a gun if you can't hit your target with it. So if you can't shoot a double action snub nosed revolver accurately, it will make a poor choice to carry. If you can't shoot a long heavy trigger (as many small carry guns have) well, it's a bad choice for you. Selecting a gun with a lighter or shorter trigger and a manual safety may be a better choice for you. Most people shoot large guns with quality sights better than small guns with little nubby sights. It is now you have to start making serious decisions. Remember decision 1, that choosing a gun you can carry all the time is better than having a larger gun you shoot better that you don't actually carry.
4. Once you've picked a gun that you can carry as often as possible, have a good holster and you can shoot well, we can focus on the next steps. Up next is being able to draw a gun and aim it on target quickly and safely. Being able to do this is mostly training, but through practice you will find there are certain guns, holsters and carrying positions will offer for faster draw times. What works best for someone else may not work best for you. Spending time actually practicing your draw will be the only way to find out what works best.
5. Next we have a duplicate entry from my prior hierarchy post, selecting a gun that allows for proper use at night is better than one that doesn't. Since crimes often take place at night, having something like an attached flashlight, a laser and night sights can all offer an advantage in identifying and properly aiming at your target.
6. Next in the decision pyramid is is the selected round. As seen in this report and this video handgun rounds are less effective than rifle rounds when it comes to terminal ballistics. Any handgun is a compromise, so addressing the points listed prior are more important than worrying about caliber. But that said, a prepared person may want to select a caliber that offers an advantage, if even a small one. It's reasonable to select the most powerful or effective caliber that you can shoot well, and that meets the prior criteria.
7. Lastly, one can consider capacity. Many gunfights involve a very small number of rounds fired, but should one be unlucky enough to find themselves in a protracted gun battle, having a gun with a larger capacity is preferred to one that holds less. Having additional ammunition available in speedloaders or magazines or even loose is better than not being able to reload if you fire all your rounds. Many small guns have a capacity of 5 or 6, larger guns often hold more. But larger guns are harder to to conceal, so keep in mind prior mentions on this list.
Many people use smaller guns in summer, when they wear less clothing, and larger guns in winter, when they are often donning layers of heavy clothing. This is a reasonable approach, so apply these recommendations in order based on the season at hand.
Carrying a gun > Not carrying one
Carrying a gun safely > Setting yourself up for a ND
Carrying a gun you can shoot well > Carrying a gun you cannot shoot well
Carrying a gun you can draw quickly > Carrying a gun that is difficult or time consuming to draw
Carrying a gun you can use in the dark > Carrying a gun with standard sights
Choosing a gun that is more powerful > Carrying a gun that is less powerful
Choosing a gun that has a high capacity > Carrying a gun with less capacity