My best S&W carry revolver is a model 49
For concealability and versatility it's hard to beat a humpback. SA if you ever want it but no-snag design all the time. If you must carry in a pocket some of the time the 638 is lightweight, but not nearly as much fun to practice with as the steel 49/649s. Personally, I don't pocket carry much at all, so in a belt holster I prefer the steel guns - especially the 649 Magnum model for its even longer, heavier (just slightly) barrel
and the longer ejector rod that comes much closer to fully extracting a 38 Special case than the "regular" models. As small-frame 38 Special shooters, I think they're the weapon of choice.
I do have 2" and 3" fixed sight K frames, and CAN conceal them under some of my usual cover garments. (I've been known to use a 3" 65 in IDPA matches, with results that are about equivalent to a 4" Model 10.) But they feel (to me) like a lot more to carry than even a steel J frame, do NOT go in the pockets of any garments that I wear, and I have experienced
some hangups with the exposed hammers and some of the lightweight cover that I wear in the summer. So IMHO, K frames are not the "best" for concealed carry.
So for the
best concealed carry S&W revolver, if I could have only one, it would be a steel humpback, preferably the magnum model. That said, I'd be quite comfortable with the original 49-no dash I bought decades ago - until I come across a 649-3 (magnum, no lock) at a good price, that is.