- Any ammo that won't work in your specific carry gun. (After your gun's well broken-in, of course.)
- Any ammo that won't meet your minimum performance requirements for penetration, expansion.
- Any ammo that disallows you to effectively engage a moving target repeatedly under stressful conditions.
Pretty much anything that meets those criteria should be good enough to survive with. test your potential carry ammo and see. Everyone's situation is a bit different. Though, starting with any of the top half-dozen loads should be a good place to start.
In the past several defensive carry guns I've had, the DoubleTap JHP (Gold Dot bullet) defensive loads have been my choice (9mm 124gr +P, .380ACP 95gr).