My CCW pistol is a Sig 938 in an Aliengear IWB 3.0. That's typically what I carry backpacking. I haven't figured out a perfect way to carry with a backpack, but the Osprey auxillary pouch that clips to your sternum strap has been OK for now.
I carried a .44 magnum in Alaska when I was in an area with bears where there was a lot of brush. I wouldn't recommend that gun for much else as it is huge and extremely heavy. Not very practical for target shooting either.
I carried an XD when I expected to fall in mud or a river. It's a cheap gun that is tough and reliable. They were cheaper than Glocks at the time by a lot, but I'd probably choose a Glock due to the lower barrel if they were the same price. Muzzle flip on an XD can be pretty bad compared to others.
"Stopping power" is very dependent on what you're stopping. No sidearm is going to work well for everything. The most common choices are going to be .45, .40, and 9mm. I might switch to .45 for my EDC if I still lived in Alaska due to being a little more comfortable with it for black bears. I had a lot of close calls with them even in my yard with big dogs running loose.
The EAA Witness / CZ75 is an excellent pistol, especially for the price. They are one of my favorites for target shooting. I wouldn't hesitate to carry one, but I have a CCW and they aren't very practical for a skinny guy. If there was one similar in size to my Sig, I'd strongly consider switching to it. Beretta 92FS is another excellent target/duty pistol. The old S&W 6906 that was a common police duty pistol is great too, although I'm not sure that a used pistol is a great idea for a newbie unless you have a way to make sure you don't get hosed.
If you're planning on concealing it, the width of the gun is way more relevant than the length/size. I'm pretty skinny and have a lot of difficulty concealing anything double stacked.
I also have zero interest in anything for defense that doesn't have night sights after carrying my Sig for a few years.
The most functional pistol for defense is one that you have a ton of practice with. For that reason, I would not recommend anything chambered in 10mm, .380, .44 mag, etc unless money for target ammunition will be of no concern to you. I usually recommend a 9mm that is practical to target shoot with as a first gun because they are cheap to shoot and functional for a lot of applications. Something like a S&W Shield would be a good compromise if you intended to use it for concealed carry and aren't easily able to conceal a larger gun. Don't start with something that will teach you bad habits.