Favorite Frame (J,K,L,N)?
My vote is for the L frame. But such choices have to be within the context of the intended use. I will emphasize that what follows is my experience and are my opinions. Don't want to step on any toes, especially when I mention any other makes.
During my department's revolver era we used 38 spl, 38 spl+p and .357 magnum rounds for duty use. In PPC competition we used 38 wadcutter and 38 standard velocity lead ball for leg matches. So my frame of reference is the 38/357 bore size.
My last personally owned duty revolver was a Model 27. Beautiful revolver which I still have. It was bulky and heavy, but at the time, as far as I could determine, it was the most rugged DA revolver available in that bore size.
When my department went to mandatory carry of department issue revolvers, the revolver selected was the S&W Model 66. They turned out to be problematic. One example was when I went through the two week Ca POST rangemaster course. By the time I had finished I was on my third Model 66. A year or so after that I had been promoted and was in charge of our patrol division. S&W had recently introduced the L frame series. Bill Davis, who was a record holding PPC shooter, had been a CHP shooting instructor for years, and had bought out a LE supply business and combined with it his Davis PPC revolvers, came through and offered a straight across deal for our 66s for 686s, just pay the sales tax. I was able to come up with necessary loot and we got the 686s. I had spoken to Bill and he was really high on the 686s. Given his expertise I trusted his word. Once we got them the reliability problems ceased. While they were heavier, they were also more controllable to shoot due to the forward weight with the lugged barrel.
I liked them so much I wanted a 686 with the tall front sight which would allow for a neck hold at 50 yards for PPC shooting. I couldn't score a 686, but did get a 586. In the course of getting this revolver customized for my intended purpose (which did not come to fruition, but that is another story) I got the 586 electroless nickel plated, along with some other steps. So I was able to derive the benefits of stainless steel. I still have it and I think it is a great revolver. It may weigh similar to a Model 27 or 28, but I prefer the weight out at the muzzle and the smaller diameter cylinder. The K frame grip size and consequent length of pull fits a wider variety of hands.
One thing I have read, and seen photos of, is peening of the bolt slots in N frame .357 cylinders from very rapid double action shooting because the large heavy cylinder comes to a sudden stop when the bolt engages. There has been a lot of discussion of issues with K frames and large doses of certain types of .357 magnum ammunition and the flat relief on the base of the forcing cone to clear the gas ring. The L frame slides right into the middle, avoiding problems documented with K and N frames.
Those are my reasons for my preference for the L frame. I just think it is the ideal S&W platform for the .357 magnum round.
In the interests of brevity, there are a couple of loose ends that will remain so, unless or until it is appropriate and of interest to delve into them:
-For my purposes and preferences, using Ruger Security Sixes instead of S&W K frames.
-Transitioning from S&W J frame to S&W 469, then to S&W 6906, and finally to (gasp) Glock Model 26.