I purchased 2 handguns on my 21st birthday. Since that day I have never had fewer than 2 at any given time.
My first thought was a 4" stainless medium frame .357 Magnum. Ideally a 686-4 +, although I don't have one. My 66-2 could easily stand in for it. Light .38 mouse fart loads to heavy .357 barn burners would cover just about anything I would need. Can be carried concealed too.
However someone in this thread mentioned ear splitting noise & muzzle flash. While not a huge concern, something with a little less "shock & awe" might just be worthwhile.
My 3" 625-4 seems just about right. .45ACP for most duties, .45 Super if I need something more. It too can be carried concealed, and I occasionally do. Moonclips are a plus also.
There are many fine autos out there, but if I down to one handgun, I'm going to presume that whatever put me in that situation (finances, gov regs etc) is also going to limit access to spare or replacement mags, recoil springs etc. A quality revolver is much less likely to need replacement springs. While it is possible to damage a moonclips, in 25+ years, I never have, and the 625 works fine without them. Besides, I already have a couple hundred.