The best is the one in your hand. I teach personal defense. You need agun you will carry. When I lived in the desert I carried smaller guns than I do in moderate climates. Lite guns tend to be taken along when heavy guns are left at home.

The best gun is the most powerful gun you are willing to take with you all the time

On duty we still carry .38 S&W revolvers and I have one of Taurus's UltraLite's in that caliber (+P) that is always with me. I also have a 25 ounce 45. It goes along a little less often. There are some really small and light 9mms, but if you are going to carry more weight then don't kid yourself. The 45 is more gun. One shot stopping power is irrelevent. Empty the guns entire capacity into him quickly.