Since this thread is about "best revolvers for concealment" and not a variety of other handguns large and small, the J-frame snub nose .38 easily takes first place.
As mentioned already, these guns, for most people, are quite difficult to shoot well. To the first-timer or the inexperienced handgunner, these guns are not comfortable to shoot and trigger pulls are heavy, but one can adapt to these "drawbacks" to the point where they are no longer shortcomings.
Sure, you can add bigger, more comfortable stocks but you may as well get a larger revolver if you're going to lessen the concealment factor. You can change out springs and reduce the reliability factor but, you'll have that lighter trigger pull you "needed".
Or you can take an unmolested .38 J-frame snub nose right out-of-the-box and learn to shoot it. If one is willing to put in the time, effort, and ammunition expense to do things right, these guns are easily the winners in the category of best concealment revolvers.