I liked the 2.5" 637, 638, 642 that S&W made for a short time. Pinned front sight, slightly better ballistics, longer radius, 1oz more & a bit easier to shoot. What's not to love?
They haven't made mine yet....I want a current production no-lock standard 640.
Other than that, probably a no-lock 642. 340 M&P's are pretty nice and no-lock, but I really truly dislike night sights and they are way overpriced. I also liked the run of two-tone 442's with the black frame and stainless steel barrel and cylinder from a couple of years back, but they did have the internal lock.
My #2 would have to be the M-36-6 adjustable sight guns.
Then again...If we are talking about "ANY J-Frame" then the entire Kit Gun line is very appealing. Whats not to like about a compact, light .22 rimfire revolver?
I've owned plenty of J-frames and found that the larger frame S&W's are better for all things but one - concealability. So I now own only one J-frame, with it's primary purpose being CCW. This older model 42, with Eagle SS grips fits nicely in a front pocket and has the added piece-of-mind of having a grip safety (not that it is really needed with the heavy trigger pull). A model 442 would probably serve equally as well, but wouldn't look as good IMO.