Nothing much wrong with a 1911 in .45 ACP. A Glock 19 or S&W M&P 9 Full size, provided I have premium ammo available, is also a good choice. My ammo of choice in 9mm is Winchester RA9B or RA9TA, or equivalent from either Federal or Speer, in its Gold Dot line.

Someone mentioned consideration for whether the need is a surprise or not.

Folks, if I know about it ahead of time, then I am going to simply avoid the situation entirely. If I know it is coming, and cannot avoid the situation entirely, then an M4 or even an M1A with plenty of spare magazines would be primary.

Recall the woman who noticed that a Texas Ranger at the party she was attending was wearing his pistol. She asked the Ranger if he was "expecting trouble." The Ranger responded, "No, ma'am. If I were expecting trouble, I would have brought my rifle."