I don't know if this is true anymore...but according to a few different reports that a LEO buddy emailed me the round with the most kills (hand gun) is a .22LR. Little bastard gets inside you and just makes a mess. But you have to hit center of mass.
Buy what fits your hand the best...and make sure you are comfortable with it. I had a Taurus 9mm for a while. Absolutely hated it. Trigger felt like you were stirring soup.
Don't know if it still available but my wife used to carry a Taurus 605. Tiny little 5 shot .357. Took some range time to figure it out. If I remember right has a front sight and a trough for a rear sight. Barrel length is 2" so at anything beyond "self defense" range its accuracy is horrible. But once you figure out how to hold it I could put all 5 rounds (hand loaded, juiced up .357) in a 2" group at 21'. Commercial ammo made it much more manageable. Wife used to love to load load it with my hand loaded .38SP rounds. Used to buy "bad guy" targets, with any .38SP she could empty it into the "head" of the bad guy...with commercial .357 all 5 rounds would end up in the center circle of the "bad guy" target.