Aim small, miss small.
I like .45 for bringing hitting power for a handgun. There’s just something about a .45 that reaches out and says ‘MERICA!!! For a rifle, any rifle round better than a .22Lr should get people’s attention.
Really it’s subjected to what one can wield effectively. Training makes it all the more effective.
The key is to have one available at the moment of truth. The state of California made it impossible for the poor murdered souls. Whatever you can keep closest to you or on you is best.