5.56

Your primary concerns in preventing injuries to innocents are accuracy and adequate penetration. Missing and directly shooting someone is a far greater concern than over-penetration and bullet effectiveness varies wildly with shot placement. The important body structures are buried deep within the torso and bullets need to be able to penetrate deep enough from any reasonable angle to damage them. Under-penetration results in ineffective hits even with perfect shot placement. The more rapidly you incapacitate the attacker(s) the fewer shots both you and the attacker(s) can fire and the safer everyone is.

Since long guns provide much improved accuracy vs. speed, terminal potential, and soft armor penetration I would choose carbine or SBR AR15s with a quality load.

If you want to stick to pistols then 9mm with a load off the list posted earlier. I believe that 9 is the best available compromise between accuracy, speed, capacity, cost, and terminal potential.

No matter what you choose prevention and proficiency are most important. Get something decent then focus your firearm resources on training.