Although I usually carry a 40 SIG, if I could only have one handgun, it would be the S&W 65 with a 3 inch barrel and round butt. This is probably the most versatile handgun ever made. It will do almost everything well although not great.
The mid sized frame, fixed sights and full length ejector rod make this one of the finest, reliable and accurate powerful combat weapons ever made.
To the uninitiated or those that have never had the good luck to have the chance to sit down with the retiring and aging breed of past law enforcement gunfighters the 65 looks like a plain jane run of the mill revolver.
To those that know what they are doing, or are lucky enough to have talked with those gunfighters, the 3 inch 65 is a very efficient defensive combat machine that can easily hold its own against any other handgun including all of the modern semi autos in any defensive caliber. Prior to the big switch to autos the 3 inch S&W 13 (the 65s blued version) was the FBI issue gun and it served them well for many years.
The S&W 908 would run a close second.