For most of the past few years that I've owned guns my answer to that question (and usually to the one gun, not one revolver, question as well) would have been a resounding 3" S&W 65LS. I love mine. There is just something about a 3" K-frame that is more than the sum of its qualities (and its qualities are quite positive), and the .357mag chambering is probably the most versatile out there.
However, the more I shoot my S&W 57 (6") the more I love both the gun and the caliber. Now, an N-frame is too big to be an only revolver (what happens if I ever manage to make the move to a carry state or somehow MD becomes one?). So, given my newfound love of the .41mag (I think I may like it better than my old favorite caliber, .357mag), I may need to pick myself up a medium framed .41mag (a Taurus Tracker) either in a 4" barrel or cutting it down to 3" so I can make a more informed answer. Heck, since I reload all I have to do is pick up some .41mag dies and I can get .41mag for about the price of the cheaper factory .38spl. I suspect that in a more suitable size to be an only revolver the .41mag would kick a bit more, but I also suspect that I will still love it. I think it would probably finally be the gun to beat out my S&W 65LS as my "one gun" or "one revolver" gun/revolver.