I voted for the .40 but there were only two criteria given.
I carry, on various occasion and at my bedside (or handy, in any event), several, from a .380 PPK to a .45 with variations of .38 S&W revolver, .357 SIG, FN5, .40.
It take only one shot at best, from any of these. If I miss, it takes as many as I can shoot till I hit.
Not too be flippant about the question but based, on the thought, if it is volume, I would choose the 20 rounds from my FN5 plus the three other magazines loaded and handy.
Based on the thought of power I would choose the .45 and one bullet.
Blessings