In light of my earlier post, I think I would have to go with a Ruger GP100 .357 Magnum, 4" barrel, SS finish (or a pre-Clinton style S&W 686). I dont own one; but then again, Im not forced to own "just one" handgun.
I honestly believe my XD-45 ACP is particularly well suited for many occasions given its effective caliber, reliablity, durability, mag capacity, its effectiveness during rapid fire vs. a revolver and therefore, it is my main "go to" weapon. Plus, the XD is simply one of the most cost-effective weapons out there. But, the revolver does offer less need for "trickery," so to speak.
Regardless, this is a tough call. I cant necessarily say the revolver is the most "ideal" handgun to own, but it may just be the simplest and most practical (if it is indeed the only handgun you can own.) Now, if you were to say "only one weapon," I would bypass all handguns for either the 12 ga. shotgun or the AR-15 rifle.