What is the best ammo that you can use for guaranteed one-shot stopping power against an attack? Trick question...
The answer is that there is no right answer!
This is because there are too many factors involved in attacks. The first one that cannot be overlooked is your attacker's motivation. A person with strong determination and will can fight through a lot of suffering. Unlike you see in the movies, a person that gets shot does not go flying backwards 10 feet. In fact, it is not at all uncommon for police officers to believe they missed hitting a suspect simply because their actions did not appear to change course.
Shot placement is another thing to consider. While you cannot rely on shot placement to finish the fight, there are simple physical laws at play. Which do you think would have a higher probability of stopping an attack; a 9mm round in the hand, or a .22 caliber in the head? The answer is obvious. Shot placement counts for something, just not everything.
Another factor is what type of substances the person may have been using. Some drugs can cause a person to not feel pain, even in extreme cases. I personally know of a situation in law enforcement where a suspect under the influence of PCP lethally attacked a group of officers. They responded with multiple shots. When it was all said and done, he received 6... yes, I said SIX... rounds of .45 caliber hollow points in the heart. It then took a pile of officers to hold him down for approximately another 30 seconds before the fighting was over.
Does this mean that you absolutely cannot stop an attacker with one round? Not at all. There are certainly a lot of documented cases where one shot stops the threat. A very good friend of mine experienced an attack in his home when a male strung out on meth kicked in his front door and made his way down the hall. One .357 round later, the fight was over. My friend was now convinced that this round was capable of finishing any fight with one shot.
My response to him was the same one I have for anybody that buys into the myth of one-shot stopping power, "Are you willing to bet your life on it?"