Great thread!
To the OP's question: K's - no doubt.
My collection of S&W has clearly gravitated toward the K frames. I have 2 J's (60-9 & 34-1 4"), 1 L (686-4+ RB), and 4 K's (64-5 4", 10-5 4", 10-4 3" & 66-4 2.5"). I shoot the K's, especially the 64-5, more than any of the others.
In my case, I shoot all the K's listed above about equally accurate out to about 50'. Now that I think about it, I shoot all of them, including the J's and L, about equally accurate to 50'. I get about a 3"-4" group at that range.
I used to have a 686-1 6" that I traded for the 686-4 4". Both were/are equally accurate at 75' - which is not that great. For me, the little circle on the target is kind of fuzzy at that range. So I think the limiting factor is my eyesight. My 64-5 is only slightly less accurate at 75' owing, I think, to the sights.
Here's a couple pics. The 36 had finish issues and got traded against the Model 60-9.
Right now, I cannot honestly think of another S&W, or barrel length that I 'have to have'. Although I will note that everybody tells me to add an N frame. Also, I'm thinking a 3" 64, 65 or 66 might be nice (but pricey). And a 67 might be nice too. You can never have enough of these!