A fairly easy one. S&W 686 3" 7 shot. Ask me tomorrow and it'll be 4" for overall utility, but I am so infatuated with my 3" 65 Lady Smith for it's "just right" size/usefulness, that I'd like any "only one" to be at least nearly as compact/carryable. In any event, load it down (.38/.38+P) or load it up with "regular" or hot mags, still one of the top manstoppers for SD/HD, and can do a yeoman's job of hunting or defense against dangerous 4 legged critters if called upon in a trail/camp/survival setting. Though not the optimal "brown bear and up" round, I'd still rather have it with me in such a circumstance (properly "loaded" of course) than any of the autos (10mm being close, but still...) and makes more sense as a daily all 'rounder than anything in a revolver starting with a .4__.
Btw, this is a revolver choice, generally, because of the "go bang everytime" simplicity of function/reliability of operation, related low maintenance and ruggedness of construction.
Second place: hi-cap .40 auto. I'm thinking something along the lines of the Springfield XDM, Beretta PX or Smith M&P. An added bonus, some of the .40s out there can be paired with a relatively easy/quick.357 SIG changeout for when that might make sense. This (specific auto selection) might be a 10mm choice of some sort--being better able to fulfill the "wilderness" role suggested above making it more an "all rounder"--if ammo were as readily available.
All in all, a mid-frame .357 DA revolver is very hard to beat for "one gun" utility. If parameters were limited to more of a "primarily urban" setting and the user was not an "outdoors type," which I am, or if making such considerations as wilderness/survival a lower priority, it'd be one of the .40 autos.