IMHO, a .357 Magnum revolver with a 4" barrel is the most versatile handgun you can have. Small enough to carry, big enough for hunting or use as a service sidearm. .38s are fun to shoot, but .357s are powerful enough for game up to deer at short range with proper shot placement.
My choice would be a stainless steel Ruger Security Six. The *Six guns are easily field stripped, rugged, and have smooth actions once they're broken in. They aren't as slick as a Smith K-Frame, but are a bit more rugged. The *Sixes are lighter than the GP-100, so they're easier to carry. I'd take the Security Six vs. the Service Six (which is what I have now) because I'd like adjustable sights.
The absolutely perfect "only one" gun though would be a Security Six with the grip from a GP-100. That would be the cat's a$$.
I really need to get one.