OP, the best gun for any purpose is the one (1) that you'll practice with, with is the same as saying that you are super, super comfortable shooting, and (2) that you will have with you when you need it. This is true for hunting, targets, and self defense.
People are different. I'm a big guy but I don't like firing the .45. For reasons that no one else can understand but I really feel, I hate to shoot the .45, so I don't, so it's fairly useless to me. The .40, I like, and I shoot. Some of my 12 gauges I love, others I haven't shot in years and the only reason I haven't sold them is laziness. Everyone but the nuttiest of the gun nuts has this experience: what you like is what you like, and you shouldn't try to talk yourself out of your own preference.
If you find that a 9mm really attracts you because it's easy to shoot, high capacity, and cheap on ammo (all of which are very much true), you should embrace it. The military uses 9's for precisely those reasons. If you are not going to fire the shotgun much at the range, and won't have it in an accessible place because it looks alarming and is hard to keep out of sight, then all that power is useless to you.
Conversations about one-shot stopping power are good to have but for most practical purposes those conversations are only for snipers and hunters. In a self defense situation -- and here I'm talking from hundreds of police investigations and police after-action reports, not just personal opinion -- your aim will probably suck, you will not have the time or wits about you to reload, and what will either incapacitate or scare off the bad guy is a fusillade of shots, not your first shot. Average number of shots fired to stop or scare away the bad buy is 3, average shots fired to lethality is above 6 (!), with any handgun (those are shots fired, not hits) (I don't know the equivalent shotgun numbers). So if self defense is really what you want, you need to get a weapon AND train on the weapon to fire many times quickly. With handguns, multiple "double tap" or three-shot bursts are the most popular training methods. With shotguns, it's typically a 3-round salvo. Be honest with yourself about whether you can handle this type of shooting with a given caliber. I can't reliably do this with a .45 but I can with a .40 or 9mm, and most ranges are happy to let you train this way. I can do it with a shotgun but the ranges near me won't let me do it (rapid fire shotgunning freaks some people out), so I have to do it outdoors, which means I don't do it, which means it's a useless thing that I can't rely on. You see my theme here.
This is a long way of saying that in a practical situation, a 9mm is a solid choice, and it's by far the best choice if you'll actually practice with it more than you would practice with the other choices. I don't actually own a 9mm, so you can rest assured that I'm not saying this because I'm a booster of it; if I were to recommend one, I'd actually recommend the .40 for the same reason most police departments have moved to it (more lethality than the 9mm, more capacity and easier to carry than the .45). It's just that I have to admit that the 9mm is easier and cheaper to shoot than the .40, has cheaper ammo, holds more, has a lot more people who really say they like to shoot it, and (at least last time I checked) has more bullet options.
Very good post! As you say, it is what the individual shoots well that matters.
I have friends who consider me their gun expert. When they are trying to buy something they always just try mimic what I buy. I always tell them that there is no one right answer when it comes to these things. Because I have come to prefer certain things does not make them "best", just best for me.
A recent example was where a friend told me that he was getting a concealed carry pistol. I told him I am in the market myself. He asked what I was getting and I told him I was after a Sig P938 SAS because I have found that the only way I carry regularly is if I can drop it in a pocket. He instantly pronounced that this is what he would get as well. I asked if he knew what it was. He had no idea. He just figured if I was going to buy one it had to be the best.
I told him I thought it would be a terrible choice for him since he does not normally shoot a 1911 he will not have the motor program to ride the safety. He is also the size of bigfoot. I am not sure his thumb could even reach the safety on a P938. This guy could drop a glock 19 in his front pocket and barely notice, so why not carry something that carries more than 6+1?
To the OP: When it comes to guns, bows, cars, computers, phones, etc, etc, etc, people always want to be happy with what they bought. If you ask what is best 90% of people will tell you to get what they have. A good way to filter those out is to ask what else is comparable. A shooter who has a lot of experience with a lot of guns and an open mind can usually name you 5 good guns in any class that all work very well. A fanboy can name only one. In their mind nothing else comes close. I can't stand fanboys.
The other issue you run into is standard answers. The problem with the internet is that lots of people like to be experts. even when they arent. If they don't actually have the knowledge they will just regurgitate the opinions of others.
When people on this board and others ask what .22 they should buy 10 people chime in and say "Ruger 10/22" before they even ask the OP what he is going to do with it. Shotgun? Remington 870 or Mossberg 500. Handgun? Get a Glock, Springfield XD, get a 1911 (which means what since there are 5000 flavors of 1911). All these answers appear without anyone asking the OP what he plans to do with it. They are all fine guns but not right for every purpose or every shooter.