If I knew I was going to be in a life and death fight but didn't know any of the details, I would rather have either a 10mm or a 357 mag revolver, with plenty of reloads or mags. Of course a 12 gauge shotgun or an AR-15 would be even better.
On the other hand, I would probably choose a Browning HP in 9mm as the gun. It is interesting to try to reason out the best gun cartridge etc.
I always thought it interesting that even real pros like Bill Jordan, William Fairbairn and Frank Hamer had different ideas. I once read that Frank Hamer was in over a hundred gunfights and he preferred the .38 super.
In the LONG run, the best gun may just be the one you still have ammo for!