There is a whole lot of BS, conjecture and misconception about handgun ammunition, from caliber to bullet type, velocity, etc. and a serious amount of what researchers call "anecdotal" information. Having spent a fair portion of my life in search of the "perfect" handgun and caliber because I buckled one on with my pants every working day for a long, long time (and mostly still do) I am reminded of the comment from an old Souix indian in his 90's who as a young boy had been a "horse holder" at Custer's last stand. When asked what "really" happened, he said "People who were not even born then have written books about that battle. Me? I was there, and all I remember is a BIG cloud of dust!"
Handguns do not have "stopping power"...stopping power is a 1964 Ford Galaxy at 60 MPH! Handguns are "hole punchers", and really not very good ones at that. Big holes work better than small holes, more holes work better than fewer holes, deep holes are better than shallow holes, but accurate holes are best of all. Now, if we can place a big, deep hole accurately upon demand, we are approaching the limits of what the handgun is able to do for us.
Hollowpoints and/or other types of "controlled expansion" ammo are a means for us to make a bigger hole, if they work. Today's defensive handgun ammo usually works pretty well, but it is not perfect. Too much emphasis upon expansion can limit penetration capability and vice versa. In addition, real world conditions may place our three dimensional human target behind cover/concealment, moving, and in low light, etc., conditions far different from those encountered in a controlled range environment. The MOST important things in personal defense are awareness, mental conditioning, tactics and skill with the equipment you have with you when the event happens.
However, because the handgun type, caliber and ammunition are things we DO have some control over (unless we are LEO's or Military) we tend to get...IMO...overly obsessed with the smallest details and end up spending far too much time on things that really don't matter very much. Think about the term "one shot stop" for a minute. What, exactly, does it mean? If the impact from the bullet physically "knocked" the assailant down, it would knock the shooter down when he fired it! If the assailant is drunk, or drugged up, or enraged, he may barely feel the impact at all. If I stick you in the ass with a pin and you jump 6ft., did the impact of the pin "throw" you that far? How about a drunk at a party...what if we stick him in the ass with the same pin? Can you say "delayed reaction"?
If this is the case, we must depend upon a hit to the "control center" (brain) or "support structure" (spinal chord) or the "hydraulic system" (heart or major artery). In a dynamic situation, perhaps in low light, perhaps with multiple assailants, careful placement of shots to targets like the head or spinal chord is a luxury we will probably not have...certainly we should not plan on it. You may very well be able to place only a single hit on a ducking, weaving opponent for several shots fired. This leaves us with shots to the center of the body where the major vessels, arteries and heart all connect. On a large man a shot through the upper arm may have to travel over a foot through the body to reach a vital spot, and after it does so, a determined adversary can continue fighting with relative efficiency for anywhere from a few seconds to a minute or more. A minute is an absolute eternity when you are fighting for your life!
What does all this say about choosing a defensive handgun and /or ammo? Simply this...choose a RELIABLE handgun that fits and works well for you, and allows you to hit what you shoot at within reasonable limits. (I want something that will shoot into 2 1/2 in or less at 25yd. if possible) Load it with good quality ammunition that has been proven to work on the street and works reliably in your gun when you shoot it. Then train like Hell!
Because there is no guarantee that my ammo will be able to expand, and because it is an accurate round that is easy to shoot, and has worked for me when I needed it, I tend to prefer the .45a.c.p., but I can live just fine with any of several others. I can use the 9mm Parabellum if I must, and while I am not its' biggest fan, 15 or so rounds of WW 127gr. +P+ JHP can certainly take a lot of worry off my mind.
The "Best" caliber for self defense is exactly what Hawkeye said, but without the other skills and attributes necessary it won't matter a whole lot.