While I believe any round will kill, I voted for the .32 ACP, I carried a Beretta Tomcat for years before I got a Kel Tec P3at.
My shooting buddies and I got into this discussion and here is a good exercise for you guys.
For $10 million dollars (and lets say you definately need the money, or your child need a special surgery to live and this is the only way to pay for it) you need to take One Shot in the COM from let us say 25 feet away, and you must be alive to collect the money. You pick the caliber and the weapon, and once you are shot you are immediately rushed to ER of a top hospital to remove the bullet.
Most guys ruled out all centerfire rifle rounds, all the shotgun buckshots round regardless of the gauge, and handgun rounds 9mm and above.
I personally voted for Birdshot, .17 .22 short, long and .22 LR coming out of a pistol only.
the .22LR, .32 and the .380 rounds were hotly debated. Some feared the lowly .22lr for it's ability to bounce around in your chest cavity.
I'm not saying you cannot kill a human with these rounds, but I do have confidence that with proper medical attention, there is a real high, one-shot survival rate.