These threads always generate posts where people list the round they think is best but with very little explanation for the reason why.
I think to go about selecting a self-defense cartridge, you have to at least form a belief about what causes incapacitation.
After you form a belief about what causes incapacitation, you have to make a decision about what situations you want to be prepared for, how much of the bell curve you want to cover with your firearm and ammunition. On one end of the spectrum is a frontal shot on the skinny guy wearing a T-shirt.
The skinny guy is 11" from front to back - not much muscle, a frontal shot with a 95gr projectile that penetrates 8" to 9" is going to reach vital tissue.
On the other end of the bell curve is having to take a cross shot on a barrel-chested, muscle-bound thug wearing a jean jacket over a shirt, and a T-shirt underneath that. Can those same rounds go through a jean jacket, a shirt, a T-shirt, a bicep, another layer of T-shirt, another layer of shirt, another layer of jean jacket, another layer of jean jacket, another layer of shirt, another layer of T-shirt and still reach vital organs?
Probably not.
Maybe you own a messenger service in Minneapolis and you drive around all winter and your primary concern is a car jacking. And you determine you need to be able to shoot through your drivers side window and stop a car jacker. Well then you want a bullet that can go through glass, heavy clothing and still reach vital tissue. Find the cartridge that does that.
Maybe you own a house on the beach in Florida and you sit on the beach all day but you want to be able to deal with muggers or thugs from the bad neighborhood down the way... so you're probably not going to be shooting through drywall, car doors or heavy clothing... just shooting into an assailant who is wearing a T-shirt or even bare chested, so find a bullet that reaches vital tissue under those circumstances.
In these "Best Self Defense Ammo" threads people are arguing past each other and advocating different rounds because they're not starting from the same understanding of what constitutes a good self defense round to begin with.