If it has to be only one, a K Frame Smith, the 10 is the classic in .38, the 65 if you want .357 ability. They aren't sexy wonder gun autos; but they've worked well for the last 114 years.
My grandfather carried one as a police officer from the 20s to the 50s. I carried one when I came on in the 80s.
I carry a nickel 2" Model 10 every day.