I guess it would depend, what I have access to, where I'm going, with who, and what they're capable using.
Depending on circumstances, 9mm, 12 gauge, 7.62x39, or 7.62x51. Depending on the range, they're compatible with my available hardware to send a good dose of "Leave me the ____ alone please" that way quickly and in volume.
If I had semi-automatics in 5.56 or 7.62x54R, I'd throw that on the list, but I only have bolt actions so, not for me.
Honestly, if you've got a reasonable quantity available, and a reliable weapon that works with it, aside from balancing out range and weight (mostly of the weapon required to fling it) I'd think most anything reasonable would suffice. Competence and practice so that you can get the rounds out efficiently and effectively should the need arise would most likely trump the particulars of one over another.
SHTF isn't like war where you can end it by getting the right bad guys (even then, it's not that simple.) You really just want to get away from other people, particularly the wrong kind of people. If I'm engaged in regular exchanges of gunfire, it's only a short matter of time before my luck drops to zero.
My perspective anyway.