Ever since I started answering this question (in multiple threads on multiple boards over a long time), my answer has always been the same: 4-inch stainless GP-100. And I'm pretty sure that's still my first choice.
However, now that I've started reloading, I might be tempted to switch to a 4- or 5-inch stainless .44, reason being that it has a higher top end for hunting, but can be loaded intermediate for defense and very light for plinking. I'd be happy with a 629 or Redhawk.
Much as I like my semi-autos, if I could only have one handgun, it would have to be a revolver for the versatility.