You are going to get a lot of advice on this question, and it will vary considerably.
First, there are those who will insist on a revolver, and there is certainly nothing wrong with that. Revolvers are simpler and therefore have fewer things to fumble with (like manual safeties and having to rack the slide to load). The downside is (usually) fewer rounds available and they tend to be more bulky. So depending on how you intend to carry that could be a problem. I don't have any revolvers but there some excellent choices out there including "hammerless" designs (they actually do have a hammer but it is recessed which means there is nothing to snag on your draw).
Next there are semi autos with manual safeties. I have two of these. Not a big deal if you practice with it, clicking off the safety becomes second nature. But if you are not going to be practicing much with it you may forget to disengage the safety when you are under pressure, and nothing says "oh ****" like a pistol that goes "click" when you want it to go "bang". The advantage is you can comfortably carry one of these with the safety on and a round chambered. Capacity for a semi auto is stated as the capacity of the magazine. So you may see, like with my favorite carry, a KelTec P32, that it will be stated as 7+1, which means 7 in the mag and one in the chamber. But the P32 does not have a manual safety, for that reason I don't recommend carrying it chambered. This means you will have to draw it and rack the slide to put a round in it.
There are other safeties for semi autos. Glocks do not have a manual safety. Instead they have a "double trigger" which means putting your finger on the trigger disables the safety and it will not fire if chambered and dropped. I have one of these, I carry it in a Blackhawk holster that covers the trigger and has a finger release button you need to depress to draw it. Now, that may seem like a pain, but then it comes to practice again. When you go to draw you should have your hand around the grip and your trigger finger will be straight forward NEXT to the trigger gaurd, which, with the Blackhawk is going to put your finger right on that release. So the holster becomes your manual safety. You can carry it loaded +1, as soon as you clear the holster you are ready to go.
The downside of the Glock for me is that it is bulky. This is largely due to the magazine being a "double stack" which means increased capacity, but a wider grip. I'm not a real big guy and if I try to carry mine on my hip with just a t-shirt I am "printing", which means you can see the shape of the pistol thru my shirt. Not exactly "concealed".
Recently I have been considering the Springfield Armory XDS in .45acp. The XD is a similiar design to the Glock, but has a grip safety, which means when you grab it you disengage the safety lever on the back of the grip automatically. It also has a manual safety, which I find redundant because it has the same double trigger setup as the Glock. The downside of this pistol is it is a "single stack". The S stands for slim, they accomplish this by having all the rounds in the mag straight up and down. Without an extended mag (which for me defeats the whole idea of concealed anyway) it holds 5+1, which is fine for me because that is 6 rounds of .45acp. That's a lot of stopping power, and the pistol is so narrow and light that it carries better than my Walther P22.
Next will be caliber. There are those who will tell you you have to have a big round, something like a .40 or .45, some will even suggest a 9mm is not enough, but I disagree. The right caliber is the one you are carrying, every time. If you get a big 44 Magnum revolver you will not carry it, which means when you need it it will not be there. At least initially, disregard caliber until you find something you can comfortably carry. Hence my .32acp. It weights 6.6 ounces empty, the lightest carry pistol on the market. Mine has a pocket clip on it, so it sits in my pants pocket, all you can see is the clip that makes it look like a pocket knife, and it is so light I often forget I have it on me. If you want a little more power, they make the same one in 380.
Statistically there is no such thing as a one round pistol caliber. People will say you need something big to take your opponent down with one shot. In reality this doesn't happen much. Keep in mind you are going to be close. Defensive shooting happen at 21 feet or less, what is considered contact range. Shoot someone from farther away and your self defense argument goes out the window. Small calibers such as 22lr, 25acp (a horrible choice) and 32acp have a kill ratio of about 1.3 rounds, which means you will be shooting twice to beat the stats. This is better than intermediate rounds such as 380 (which is really a 9mm with a lighter bullet and a little less powder, a 380 is 9x17mm, a 9mm is 9x19mm) is more like 1.6 rounds. The reason is that the little ones are usually used at a closer distance so there is less chance of missing. Even big rounds like 38 Special, .357, .40 and .45acp have a ratio of 1.1. They hit a lot harder, but statistically you will still need more than one round.
Which brings us to the last variable: follow up shots. The smaller and lighter the pistol the more recoil you will feel, which means more difficult to get back on target. Likewise the more powerful the round the more recoil, same thing. I want easy conceal, decent power, and easy follow up, which is why I like my P32. The 380 version of it is the same size but has more "snap" to it, which means harder to get back on target. The 380 holds 6+1, the .32 holds 7+1. Damage wise at close range there is very little difference between .32 and 380, but the .32 is faster back on target and carries one more round.
A lot of info, I know. But it is an important decision and too often you will just get "I like my......." without any pros and cons. My subcompact Glock is a .357 Sig, which is a hell of a round. The pistol is light but bulky, so I rarely carry it unless it is jacket weather. I love my Walther P22, easy to carry but a bit underpowered, although I overcome that with some hot rounds. It is easily my favor pistol to shoot and is my prefered range pistol. My P32 KelTec is light, hides extremely well and gives me enough power to make me comfortable carrying it. And like I said, the best caliber it the one you have on you.
