I have revolvers by S&W, Taurus and Colt (I own one new production Colt, some years back I had an old Colt Police Positive Special). I have never owned a Ruger, though I've shot a few Ruger rentals over the years. I've come close a few times, but I don't love the looks or the trigger of those I've tried.
If I could only have one manufacturer, it would be S&W. Great triggers, well made (even most of the new production IMO), and a lot of different options. If we are talking only one manufacturer, you have everything from lightweight snubs to some of the largest and heaviest revolvers made, and you have pretty much any caliber that revolvers are chambered in, you have steel, aluminum alloy, more space age alloys, and even polymer. No one has the choices that S&W gives you other than Taurus (and even there you lose the more space age alloys and .45LC). While Ruger loses the alloys, you do have polymer if you need a lightweight revolver, and you add the single action revolvers, but I prefer S&W.
Colt makes a great revolver, and I love the DA trigger. However, being as used to S&W and Taurus trigger resets and cylinder releases as I am, it would be hard to make the switch in defensive guns. If I was starting over and I wouldn't need to retrain, or only using revolvers for range guns, I'd consider a Colt here (at least if we're talking only one revolver and not just only one manufacturer, otherwise, the variety of S&W would win).
If we are talking only one revolver, I'd likely go with a 3" K-frame .357mag. Small and handy enough to be concealable, long enough barrel to get enough velocity out of .38 or .357 rounds for good performance (at 2" there are only a few loadings that don't sacrifice either expansion or penetration), large and heavy enough to be a great range and home defense gun, and since it is chambered in .357mag you can use it as a woods gun (it is just enough for most wildlife defense, at least for the size bear you'll see east of the Mississippi River).
